Attached project for reproducing.

Workaround:
var column = radGridView1.Columns[2] as GridViewDecimalColumn;
var format = column.FormatString;
column.FormatString = "";
column.ExcelExportType = DisplayFormatType.Currency;

//export the grid

column.FormatString = format;

Please refer to the attached sample project and follow the steps illustrated in the attached gif file.

To reproduce:
        public Form1()
        {
            InitializeComponent();
            DataTable dt = new DataTable();
            dt.Columns.Add("Id", typeof(int));
            dt.Columns.Add("Name", typeof(string));
            dt.Columns.Add("ParentId", typeof(int));

            for (int i = 0; i < 3; i++)
            {
                dt.Rows.Add(i, "Parent" + i, -1);
            }
            Random rand = new Random();
            for (int i = 3; i < 15; i++)
            {
                dt.Rows.Add(i,"Child"+i,rand.Next(0,3));
            }

            this.radGridView1.Relations.AddSelfReference(this.radGridView1.MasterTemplate, "Id", "ParentId");
            this.radGridView1.DataSource = dt;
            this.radGridView1.AutoSizeColumnsMode = GridViewAutoSizeColumnsMode.Fill;
            this.radGridView1.EnableGrouping = true;
            this.radGridView1.EnableCustomGrouping = true;
            this.radGridView1.CustomGrouping+=radGridView1_CustomGrouping;
         }

private void radGridView1_CustomGrouping(object sender, GridViewCustomGroupingEventArgs e)
{
     
}

To reproduce:
private void RadGridView1_KeyPress(object sender, KeyPressEventArgs e)
{
    int startIndex = radGridView1.CurrentRow.Index +1;

    for (int i = startIndex; i < radGridView1.Rows.Count; i++)
    {
        var value = radGridView1.Rows[i].Cells["Name"].Value.ToString().ToLower();
        if (value.StartsWith(e.KeyChar.ToString()))
        {
            radGridView1.TableElement.ScrollToRow(radGridView1.Rows[i]);
            radGridView1.ClearSelection();
            radGridView1.Rows[i].IsSelected = true;
            radGridView1.Rows[i].IsCurrent = true;
            radGridView1.Columns[1].IsCurrent = true;
            break;
        }
    }
}

- Press some key so the grid is scrolled down, then press the down key.
- The first row is selected instead of the next one. 

Workaround:
remove this line:
radGridView1.Rows[i].IsSelected = true;

To reproduce:
1. Run the attached sample project.
2. Right-click the grid.
3. Quickly navigate to another application (e.g. Windows Explorer) before the context menu is shown & wait at windows explorer to display the menu
4. The context will be stuck.

To reproduce: please refer to the attached sample project and follow the steps in the provided gif file: 
1. Enter something in the search bar
2. Filter on some column, uncheck "All", then select a few items, then validate
3. Click on a random row on the grid
4. Filter on the same column as in 2., check "All", then uncheck a few items, then validate

Workaround:

private void radGridView1_FilterChanged(object sender, Telerik.WinControls.UI.GridViewCollectionChangedEventArgs e)
{
    this.radGridView1.MasterTemplate.Refresh();
}

Please refer to the attached sample project and gif file illustrating the behavior.
ColumnGroupsViewDefinition view = this.radGridView1.ViewDefinition as ColumnGroupsViewDefinition;

            foreach (GridViewColumnGroup group in view.ColumnGroups)
            {
                group.ShowHeader = false;
            }

Workaround: use the ViewCellFormatting and enable the back color for the header cells that are over the hidden group headers

public RadForm1()
{
    InitializeComponent();

    ColumnGroupsViewDefinition view = this.radGridView1.ViewDefinition as ColumnGroupsViewDefinition ;
    foreach (GridViewColumnGroup group in view.ColumnGroups)
    {
        group.ShowHeader = false;
    }
}

private void radGridView1_ViewCellFormatting(object sender, CellFormattingEventArgs e)
{
    if (e.Row is GridViewTableHeaderRowInfo && !(e.CellElement is GridColumnGroupCellElement))
    {
        e.CellElement.DrawFill = true;
        e.CellElement.BackColor = Color.FromArgb(234, 242, 252);
        e.CellElement.GradientStyle = GradientStyles.Solid;
    }
    else
    { 
        e.CellElement.ResetValue(LightVisualElement.DrawFillProperty, ValueResetFlags.Local);
        e.CellElement.ResetValue(LightVisualElement.BackColorProperty, ValueResetFlags.Local);
        e.CellElement.ResetValue(LightVisualElement.GradientStyleProperty, ValueResetFlags.Local);
    }
}

How to reproduce:
public partial class Form1 : Form
{
    public class Data
    {
        public bool Checked { get; set; }
        public string Text { get; set; }
    }

    private readonly RadGridView _grid;

    private readonly List<Data> _dataList = new List<Data>
    {
        new Data {Text = "abc"},
        new Data {Text = "def"},
        new Data {Text = "ghi"},
        new Data {Text = "jkl"},
        new Data {Text = "mno"},
        new Data {Text = "pqr"},
        new Data {Text = "stu"},
        new Data {Text = "vwx"},
        new Data {Text = "yz0"}
    };

    public Form1()
    {
        InitializeComponent();

        _grid = new RadGridView
        {
            Dock = DockStyle.Fill,
            AllowSearchRow = true
        };

        Controls.Add(_grid);

        _grid.DataSource = _dataList;
    }

    protected override void OnLoad(EventArgs e)
    {
        base.OnLoad(e);

        _grid.AutoSizeColumnsMode = GridViewAutoSizeColumnsMode.Fill;
        ((GridViewCheckBoxColumn)_grid.Columns["Checked"]).EnableHeaderCheckBox = true;
        
        _grid.Columns["Checked"].SortOrder = RadSortOrder.Ascending;
        ((GridViewCheckBoxColumn)_grid.Columns["Checked"]).Checked = ToggleState.On;
        _grid.MasterView.TableSearchRow.Search("abc");
    }
}

Workaround: clear the search text
protected override void OnLoad(EventArgs e)
{
    base.OnLoad(e);

    _grid.AutoSizeColumnsMode = GridViewAutoSizeColumnsMode.Fill;
    ((GridViewCheckBoxColumn)_grid.Columns["Checked"]).EnableHeaderCheckBox = true;
    
    _grid.Columns["Checked"].SortOrder = RadSortOrder.Ascending;
    ((GridViewCheckBoxColumn)_grid.Columns["Checked"]).Checked = ToggleState.On;
    _grid.MasterView.TableSearchRow.Search("abc");

    this._grid.MouseDown += _grid_MouseDown;
}

private void _grid_MouseDown(object sender, MouseEventArgs e)
{
    RadGridView grid = (RadGridView)sender;
    RadCheckBoxElement cell = grid.ElementTree.GetElementAtPoint(e.Location) as RadCheckBoxElement;
    if (cell != null && cell.Parent is GridCheckBoxHeaderCellElement && grid.SortDescriptors.Count > 0))
    {
        grid.MasterView.TableSearchRow.Search("");
    }
}

To reproduce:
protected override void OnLoad(EventArgs e)
{
    base.OnLoad(e);
    DataTable dt = NewDt();
    this.radGridView1.DataSource = dt;      
    var c = this.radGridView1.Columns["Calculted"] as GridViewDecimalColumn;
    c.EnableExpressionEditor = true;
    c.Expression = "Column2 / SUM(Column2) * 10";
    c.DecimalPlaces = 2;
    c.FormatString = "{0:N2}";

    this.radGridView1.GroupDescriptors.Add("GroupByColumn", ListSortDirection.Ascending);
}

private DataTable NewDt()
{
    var dt = new DataTable();
    dt.Columns.Add("ItemColumn");
    dt.Columns.Add("GroupByColumn");
    dt.Columns.Add("Column2", typeof(decimal));
    dt.Columns.Add("Calculted", typeof(decimal));

    for (int i = 1; i <= 20; i++)
    {
        string gr = string.Empty;
        if (i % 3 == 0)
            gr = "gr3";
        else if (i % 2 == 0)
            gr = "gr2";
        else if (i % 5 == 0)
            gr = "gr5";
        else
            gr = "item" + i.ToString();

        dt.Rows.Add("id #" + i.ToString(), gr, i, 0);
    }
    return dt;
}

Workaround:
public class CustomExpressionContext : Telerik.Data.Expressions.ExpressionContext
{

    RadGridView radGridView;
    public CustomExpressionContext(RadGridView grid)
    {
        this.radGridView = grid;
    }

    public decimal MySum(string columnName)
    {
        decimal sum = 0;
        
        foreach (var item in radGridView.Rows)
        {
            sum += (decimal)item.Cells[columnName].Value;
        }
        return sum;
    }
}

 Telerik.Data.Expressions.ExpressionContext.Context = new CustomExpressionContext(this.radGridView1);

To reproduce: run the attached sample project. You will notice the result illustrated in the provided screenshot. Although the grid is RTL, the print preview dialog show it in LTR.

Workaround:

public class CustomGridPrintStyle : GridPrintStyle
{
    protected override BaseGridPrintRenderer InitializePrintRenderer(RadGridView grid)
    {
        if (this.PrintRenderer != null)
        {
            this.PrintRenderer.PrintCellPaint -= renderer_PrintCellPaint;
            this.PrintRenderer.PrintCellFormatting -= renderer_PrintCellFormatting;
            this.PrintRenderer.ChildViewPrinting -= renderer_ChildViewPrinting;
        }

        BaseGridPrintRenderer renderer = null;

        if (grid.ViewDefinition.GetType() == typeof(ColumnGroupsViewDefinition))
        {
            if (!(renderer is ColumnGroupsViewDefinitionPrintRenderer))
            {
                renderer = new CustomColumnGroupsViewDefinitionPrintRenderer(grid);
            }
        }
        else if (grid.ViewDefinition.GetType() == typeof(HtmlViewDefinition))
        {
            if (!(renderer is HtmlViewDefinitionPrintRenderer))
            {
                renderer = new HtmlViewDefinitionPrintRenderer(grid);
            }
        }
        else
        {
            if (!(renderer is TableViewDefinitionPrintRenderer))
            {
                renderer = new TableViewDefinitionPrintRenderer(grid);
            }
        }

        renderer.ChildViewPrinting += renderer_ChildViewPrinting;
        renderer.PrintCellFormatting += renderer_PrintCellFormatting;
        renderer.PrintCellPaint += renderer_PrintCellPaint;

        return renderer; 
    }

    private void renderer_PrintCellPaint(object sender, PrintCellPaintEventArgs e)
    {
        this.OnPrintCellPaint(sender, e);
    }

    private void renderer_PrintCellFormatting(object sender, PrintCellFormattingEventArgs e)
    {
        this.OnPrintCellFormatting(sender, e);
    }

    private void renderer_ChildViewPrinting(object sender, ChildViewPrintingEventArgs e)
    {
        this.OnChildViewPrinting(sender, e);
    }
}

public class CustomColumnGroupsViewDefinitionPrintRenderer : ColumnGroupsViewDefinitionPrintRenderer
{
    public CustomColumnGroupsViewDefinitionPrintRenderer(RadGridView grid) : base(grid)
    {
    }

    protected override void PrintRow(GridViewRowInfo row, ColumnGroupRowLayout rowLayout, GridPrintSettings settings, int currentX, int currentY, Graphics graphics)
    { 
        float scrollableColumnsOffset = 0f;
        float rightPinnedColumnsOffset = 0f;

        foreach (GridViewColumn col in rowLayout.RenderColumns)
        {
            if (col is GridViewRowHeaderColumn || col is GridViewIndentColumn)
            {
                continue;
            }

            float height = rowLayout.GetRowHeight(row);
            RectangleF cellBounds = rowLayout.GetCorrectedColumnBounds(row, col, this.GridView.RightToLeft == RightToLeft.Yes,
                new RectangleF(0f, 0f, rowLayout.DesiredSize.Width, height));

            if (cellBounds == RectangleF.Empty)
            {
                continue;
            }

            if (col.PinPosition == PinnedColumnPosition.Left)
            {
                if (scrollableColumnsOffset < cellBounds.Right + rowLayout.Owner.CellSpacing)
                {
                    scrollableColumnsOffset = cellBounds.Right + rowLayout.Owner.CellSpacing;
                    rightPinnedColumnsOffset = scrollableColumnsOffset;
                }
            }
            else if (col.PinPosition == PinnedColumnPosition.None)
            {
                if (rightPinnedColumnsOffset < scrollableColumnsOffset + cellBounds.Right + rowLayout.Owner.CellSpacing)
                {
                    rightPinnedColumnsOffset = scrollableColumnsOffset + cellBounds.Right + rowLayout.Owner.CellSpacing;
                }

                cellBounds.X += scrollableColumnsOffset;
            }
            else
            {
                cellBounds.X += rightPinnedColumnsOffset;
            }

            cellBounds.Offset(currentX, currentY);

            GridViewCellInfo cell;
            CellPrintElement printCell;

            if (row is GridViewTableHeaderRowInfo)
            {
                cell = this.GridView.MasterView.TableHeaderRow.Cells[col.Name];

                printCell = this.CreateHeaderCellPrintElement(col);
                if (printCell.Font != settings.HeaderCellFont)
                {
                    if (settings.HeaderCellFont != null)
                    {
                        printCell.Font = settings.HeaderCellFont;
                    }
                    else
                    {
                        settings.HeaderCellFont = printCell.Font;
                    }
                }
            }
            else if (row is GridViewSummaryRowInfo)
            {
                GridViewSummaryRowInfo rowInfo = row as GridViewSummaryRowInfo;
                cell = rowInfo.Cells[col.Name];

                if (cell == null)
                {
                    continue;
                }

                printCell = this.CreateSummaryCellPrintElement(cell);
                if (printCell.Font != settings.SummaryCellFont)
                {
                    if (settings.SummaryCellFont != null)
                    {
                        printCell.Font = settings.SummaryCellFont;
                    }
                    else
                    {
                        settings.SummaryCellFont = printCell.Font;
                    }
                }
            }
            else
            {
                cell = row.Cells[col.Name];

                if (cell == null)
                {
                    continue;
                }

                if (col is GridViewImageColumn)
                {
                    printCell = this.CreateImageCellPrintElement(cell);
                }
                else
                {
                    printCell = this.CreateDataCellPrintElement(cell);
                    if (printCell.Font != settings.CellFont)
                    {
                        if (settings.CellFont != null)
                        {
                            printCell.Font = settings.CellFont;
                        }
                        else
                        {
                            settings.CellFont = printCell.Font;
                        }
                    }
                }
            }

            printCell.TextPadding = this.GridView.PrintStyle.CellPadding;
            printCell.RightToLeft = this.GridView.RightToLeft == RightToLeft.Yes;

            Rectangle rect = new Rectangle((int)cellBounds.X, (int)cellBounds.Y, (int)cellBounds.Width, (int)cellBounds.Height);

            PrintCellFormattingEventArgs formattEventArgs = new PrintCellFormattingEventArgs(row, col, printCell);
            this.OnPrintCellFormatting(formattEventArgs);

            formattEventArgs.PrintCell.Paint(graphics, rect);
           
            PrintCellPaintEventArgs paintEventArgs = new PrintCellPaintEventArgs(graphics, row, col, rect);
            this.OnPrintCellPaint(paintEventArgs);               
        }
    }
}

private void radButton1_Click(object sender, EventArgs e)
{ 
    CustomGridPrintStyle style = new CustomGridPrintStyle();
    
    this.radGridView1.PrintStyle = style;
    RadPrintDocument printDoc = new RadPrintDocument();
    printDoc.Landscape = true;
    printDoc.RightHeader = "Right Header";
    printDoc.HeaderHeight = 100;
    printDoc.AssociatedObject = this.radGridView1;
    
    RadPrintPreviewDialog dialog = new RadPrintPreviewDialog(printDoc);
    
    dialog.Size = new System.Drawing.Size(Screen.PrimaryScreen.Bounds.Width - 200, Screen.PrimaryScreen.Bounds.Height - 200);
    dialog.SetZoom(0.80);
    dialog.ShowDialog();
}

There should be a way for users to determine if a row passes a certain filter criteria without that affecting any data operations.

The selection mode should be as in Windows Explorer - when Control is pressed, one should be able to move the current row with the up/down arrow keys and when Space is pressed, the current row should be selected/deselected. 

This implementation can be used for the time being:
  public partial class Form1 : RadForm
    {
        protected override void OnLoad(EventArgs e)
        {
            base.OnLoad(e);

            AddGridSimpleUnbound();
            radGridView1.MultiSelect = true;

            BaseGridBehavior gridBehavior = radGridView1.GridBehavior as BaseGridBehavior;
            gridBehavior.UnregisterBehavior(typeof(GridViewDataRowInfo));
            gridBehavior.RegisterBehavior(typeof(GridViewDataRowInfo), new MyRowBehavior());

            radGridView1.GridViewElement.Navigator = new MyNavigator();
        }

        class MyNavigator : BaseGridNavigator
        {
            protected override bool DoMultiSelect(GridViewRowInfo oldRow, GridViewColumn oldColumn, GridViewRowInfo row, GridViewColumn column)
            {
                if (!this.IsShiftButtonPressed && this.IsControlButtonPressed && !this.IsMouseSelection)
                {
                    return true;
                }

                return base.DoMultiSelect(oldRow, oldColumn, row, column);
            }
        }

        class MyRowBehavior : GridDataRowBehavior
        {
            bool kbdSelection = false;
            public override bool ProcessKeyPress(KeyPressEventArgs keys)
            {
                if (kbdSelection)
                {
                    kbdSelection = false;
                    return true;
                }
                return base.ProcessKeyPress(keys);
            }
            public override bool ProcessKey(KeyEventArgs keys)
            {
                if (keys.KeyCode == Keys.Space && this.GridControl.MultiSelect && keys.Control)
                {
                    this.GridControl.CurrentRow.IsSelected ^= true;
                    kbdSelection = true;
                    return false;
                }
                else
                {
                    return base.ProcessKey(keys);
                }
            }
        }
}

Similar property is already available in the property grid and tree view exporters.

To reproduce:

Sub New()        
    InitializeComponent() 
    Dim items As New List(Of WorkOrder)
    For index = 1 To 56
        items.Add(New WorkOrder(index, "Item" & index, 0))
    Next
    Dim rand As New Random
    For index = 57 To 500
        items.Add(New WorkOrder(index, "Item" & index, rand.Next(1, 56)))
    Next

    Me.RadGridView1.Relations.AddSelfReference(Me.RadGridView1.MasterTemplate, "Id", "ParentId")
    Me.RadGridView1.DataSource = items
    Me.RadGridView1.AutoSizeColumnsMode = Telerik.WinControls.UI.GridViewAutoSizeColumnsMode.Fill
    Me.RadGridView1.EnablePaging = True
    Me.RadGridView1.PageSize = 20
End Sub

Public Class WorkOrder
    Private _id As Integer
    Private _name As String
    Private _parentId As Integer
    
    Public Sub New(id As Integer, name As String, parentId As Integer)
        Me._id = id
        Me._name = name
        Me._parentId = parentId
    End Sub

    Public Property Id() As String
            Get
                Return _id
            End Get
            Set(ByVal value As String)
                _id = value
            End Set
        End Property
    
    Public Property Name() As String
            Get
                Return _name
            End Get
            Set(ByVal value As String)
                _name = value
            End Set
        End Property

    Public Property ParentID() As Integer
            Get
                Return _parentId
            End Get
            Set(ByVal value As Integer)
                _parentId = value
            End Set
        End Property
End Class

Workaround: use standard hierarchy: http://docs.telerik.com/devtools/winforms/gridview/hierarchical-grid/hierarchy-of-one-to-many-relations

Example : if we have 5 rows in the grid and if we copy 10 rows from excel and paste in first row, only first 5 records gets pasted and remaining 5 would be ignored.

It would be great if multi-page printing is supported for grids with ColumnGroupsViewDefinition and HtmlViewDefinition.

 RadGridView, TextBoxColumn - add support for autocomplete.