CheckedItems collection of RadTreeView contains all items.

Edit: Please avoid using this collection because it has never been implemented completely in RadTreeView. In DataBinding scenarios, you can use CheckBox in the ItemTemplate of the RadTreeView. Approach can be found here =>
http://docs.telerik.com/devtools/wpf/controls/radtreeview/how-to/howto-tri-state-mvvm.html 
CheckedItems colelction can be implemented in the ViewModels, instead of using the built-in property.

The XAML team has recently reviewed this issue and will not be addressing it as at this time the team is focusing on the bugs impacting the highest number of developers. If you have encountered this issue and it is blocking for your work please contact us through the support ticketing system with details on your setup and a reference to this item.

The TreeView doesn't update the TriState of the parent item when an item is dragged and dropped.

In databound scenario the built-in checkbox and the tri-state mode are considered not supported and users are prompted to use checkbox in itemtemplate with 3state logic in the viewmodels.
http://docs.telerik.com/devtools/wpf/controls/radtreeview/how-to/howto-tri-state-mvvm.html 

The XAML team has recently reviewed this issue and will not be addressing it as at this time the team is focusing on the bugs impacting the highest number of developers. If you have encountered this issue and it is blocking for your work please contact us through the support ticketing system with details on your setup and a reference to this item.

The CheckedItems  collection is not cleared when ItemsSource is changed.

Please consider CheckedItems as not supported collection in RadTreeView. In DataBinding scenarios , use the approach described in the following article:
http://docs.telerik.com/devtools/wpf/controls/radtreeview/how-to/howto-tri-state-mvvm.html 
This way you will be able to create CheckedItems property in your ViewModels.

The XAML team has recently reviewed this issue and will not be addressing it as at this time the team is focusing on the bugs impacting the highest number of developers. If you have encountered this issue and it is blocking for your work please contact us through the support ticketing system with details on your setup and a reference to this item.

This will solve scenarios in which user wants to add new items somewhere (inner level, outside the ViewPort ) in virtualized treeview and then wants to begin edit these items (or focus them , select them). 
There is currently no mechanism to detect when the new items are ready (measured / completely generated), ItemsPrepared for example passes before the OnApplyTemplate of the RadTreeViewItem.

Tree is Virtualized and bind to colelction of DataItems.
On a button click, we add new Dataitem with IsInEditMode = true (bound to IsInEditMode of the RadTreeViewItem).
The new Item is in edit mode but the textBox in the ItemEditTemplate has no DataContext so no actual editing  can be performed.

Scenario casn be solved with adding the item, then switch its editmode in dispatcher:
DataItem item = new DataItem() { Name = "Item Editable" };
             
            (this.tree.ItemsSource as ObservableCollection<DataItem>).Insert(0, item);

            Dispatcher.BeginInvoke(new Action(() =>
            {
                item.IsInEditMode = true;
            }), DispatcherPriority.Loaded);

The XAML team has recently reviewed this issue and will not be addressing it as at this time the team is focusing on the bugs impacting the highest number of developers. If you have encountered this issue and it is blocking for your work please contact us through the support ticketing system with details on your setup and a reference to this item.

RadTreeView and RadTreeViewItem ExpandAll() method sets the IsExpanded property to True to all items even if an Item has no children to display. This causes an issue when using the DefaultImageSrc and ExpandedImageSrc properties as the last-level items will be displayed with an ExpandedImageSrc as their IsExpanded property it set to True. However, as these items have no children, they should use the DefaultImageSrc.

When the RadTreeViewItem (or the RadTreeView) is disabled and after a delay is enabled again it should go in "Normal" state. Now the control preserves its state, hence you are able to send more than one RadTreeViewItem in "Hovered" state.

The fix will be included in Q2 2015 SP.

Root level items ItemTemplate is incorrectly set after virtualization when using HierarchicalDataTemplate with ItemTemplateSelector.
This causes the second level items to use incorrect data template.

Update the radtreeview-howto-exapand-select-item-with-lod-enabled-treeview

Create localization resources for the DropActionText in the TreeViewDragVisual element. The localization resources should work as the TextDropAfter, TextDropBefore, TextDropIn and TextDropRoot properties used to work in the Legacy mode of the RadTreeView DragDrop.

When using binding to check state and dynamically adding subitem to a praticular items, the parent items check state is incorrectly set to Off (from ON) although the child item state is also On.

Marking this Declined. Binding to CheckState is not supported in MVVM scenarios. Please use the approach described here =>
http://www.telerik.com/help/wpf/radtreeview-howto-tri-state-mvvm.html

Rendering issues occure when using Hierarchical virtualization mode and bring into view.
The issue is hit in both platforms - WPF and Silverlight. 

The IsDragPreviewEnabled and IsDragTooltipEnabled properties don't work with the "NEW" DragDropExecutionMode.

Scenario: Databound TreeView with IsSelected bindings between item and container. Adding a selected data object to the ItemsSource does not fire the Selected event.

We have RadContextMenu in RadTreeView when we press the App key or shift+F10 the context menu opens as intended but it triggers the parent TreeViewItem hover effect

TreeView: When the header of an Item is set to a TextBox, the selection of a RadTreeViewItem is incorrect

The original reported issue is that the following style does not work as expected.
<Style TargetType="telerik:RadTreeViewItem">
<Style.Triggers>
<Trigger Property="IsKeyboardFocusWithin" Value="True">
<Setter Property="IsSelected" Value="True" />
<Setter Property="Tag" Value="Selected" />
</Trigger>
<Trigger Property="IsKeyboardFocusWithin" Value="False">
<Setter Property="IsSelected" Value="False" />
<Setter Property="Tag" Value="Unselected" />
</Trigger>
</Style.Triggers>
</Style>

This is not supported by the RadTreeView Selection logic. Once the trigger applies with Selected / True, the IsSelected property is Locally set via the selector's code. This means future work of the trigger will not be successful over the IsSelected Property.
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The XAML team has recently reviewed this issue and will not be addressing it as at this time the team is focusing on the bugs impacting the highest number of developers. If you have encountered this issue and it is blocking for your work please contact us through the support ticketing system with details on your setup and a reference to this item.

Change the SelectedValue implementation to allow a TwoWay binding

Allow cancelling a drag operation originating from certain items - basically the customers need to disable the drag operation on some items.
====
Here is a possible solution with handling DragInitialize
 
DragDropManager.AddDragInitializeHandler(this.tree, this.OnDragInitialize, true);

 private void OnDragInit(object sender, DragInitializeEventArgs e)
        {
            if (((e.OriginalSource as RadTreeViewItem).Header == "B1")
            {
                e.Data = null;
                e.DragVisual = null;
            }

When the FlowDirection is RightToLeft, the vertical lines of the control aren't properly aligned.

The issue is not reproducible with R3 2016 Release.

When value types are inserted in the Items collection of RadTreeView and root values are duplicated in the children a StackOverflowException is raised after a call to ExpandAll.

The XAML has recently reviewed the status of this issue and won't be addressing it in future releases.
This is a design limitation - RadTreeView does not support duplication of value types in its Items / ItemsSource collections. 
This potentially can cause troubles with virtualization / selection / expandALL features.