Current behavior:

http://admin.telerik.com/Images/editor/Silverlight/PanelBar/31-08-10/current_outcome.png

Expected behavior:

http://admin.telerik.com/Images/editor/Silverlight/PanelBar/31-08-10/desired_outcome.png

Possbility to have PanelBarItems with fixed height and other PanelBarItems that take the rest of the available height equally

Exception is thrown in the designer when threre are many items in the PanelBar control.

When using the new DragDrop logic in the RadPanelBar control, the MouaseOver state of a dragged item isn't cleared on drop. This is why as soon as you move the mouse over another item and start dragging it, you get two PanelBarItems in MouseOver state in the PanelBar.

When an level 2 RadPanelBarItem is selected initially it is stretched. Pressing the Enter key will resize it so that its children fit. 

As a workaround you can subscribe for the PreviewKeyDown of RadPanelBar and handle the event if the item should not be collapsed.

private void OnPreviewKeyDown(object sender, KeyEventArgs e)
{
    var panelBarItem = sender as RadPanelBarItem;
    if (panelBarItem != null && panelBarItem.Level == 2 && e.Key == Key.Enter)
    {
        e.Handled = true;
    }
}

Cannot dynamically change the theme of the control.

PanelBar: A custom template can not be set to a PanelBarItem, using a ControlTemplate

DECLINED: This issue is not reproducible with 2016 R3 official release.

Items to automatically adjust position when PanelBar has no scrollbar. See attached project.

DECLINED: AdjustPositionOfPanelBarItemsNoScrollBar SDK sample demonstrates an approach to customizing the RadPanelBarItems to include a ScrollBar when the RadPanelBar's vertical ScrollBar is Disabled.
https://github.com/telerik/xaml-sdk/tree/master/PanelBar/AdjustPositionOfPanelBarItemsNoScrollBar

Make expand animation to be horizontal when Orientation is Horizontal.

DECLINED: Cannot reproduce with R3 2016 binaries.

Second level items cannot be styled to look like first level items.

DECLINED: SecondLevelItemsStyledLikeTopLevelItems SDK sample showing approach to styling PanelBar with more than two levels of PanelBarItems is available since R3 2016 Official Release.
https://github.com/telerik/xaml-sdk/tree/master/PanelBar

Add an Icon property to allow displaying images in the RadPanelBarItem Header.

Declined:

Adding such property requires also adding a visual element (Image) in the Template of the RadPanelBarItems.
RadTreeView also has Image in its ControlTemplate and we have to admit this was our design mistake in the past. Such element makes the Style larger, makes the virtualization harder to support, slows down the performance.

How To Add Image in RadPanelBar:

In non-databound scenarios just add the Image in the Header:

      <telerik:RadPanelBarItem>
                <telerik:RadPanelBarItem.Header>
                    <StackPanel>
                        <Image Source="MyImage.png" Margin="5"/>
                        <TextBlock Text="{Binding Name}" />
                    </StackPanel>
                </telerik:RadPanelBarItem.Header>
            </telerik:RadPanelBarItem>

In databound scenario set the ItemTemplate of the RadPanelBar:

DataTemplate x:Key="itemTemplate">
                <StackPanel>
                    <Image Source="MyImage.png" Margin="5"/>
                    <TextBlock Text="{Binding Name}" />
                </StackPanel>
            </DataTemplate>

 <telerik:RadPanelBar ItemTemplate="{StaticResource itemTemplate}"/>

See the SDK example demonstrating those approaches: https://github.com/telerik/xaml-sdk/tree/master/PanelBar/AddHeaderIcon

Expose a property to easily control the expander indicator style and visibility.

From Q1 2016 SP users will able to use the ExpanderStyle property of the RadpanelBarItem in order to style the arrow expander.

Now if you add a content to the panelbar the content inside will receive tab navigation only once due to the items control nature of the control.

The XAML team has recently reviewed the status of this issue and won't be addressing it in future releases.
In order to make the content (items) of a specific RadPanelBarItem (X)  focusable with tabKey, set TabNavigation="Local" on X.

RadPanelBaritem's expander arrow should not be visible when there are no subitems.

XAML Team has recently reviewed the status of this item. It is considered a design limitation of the RadPanelBar control which won't be addressed in future releases.
However, users will be able to use the ExpanderStyle property of the RadPaneBarItem (from Q1 2016 SP) in order to control the arrow's visibility.