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

PanelBarItem with no Items is initially expanded when its IsExpanded property is data bound

The IsTabStop is set to false in all RadPanelBarItems and set to true in the textboxes in the Items of the PanelBarItems. When tab navigating the RadPanelBarItems get focused, in Silverlight they don`t.

When using an ItemContainerStyle on the second(non-root) BarItems, the Background doesn`t apply

Add a ScrollViewer to the PanelBarItem ControlTemplate

When the RadPanelBar is oriented Horizontally , The ScrollViewer does not work when the mouse wheel button is used.

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

If the RadPanelBar Orientation is Horizontal, a RadHtmlPlaceHolder cannot be displayed inside a RadPanelBarItem

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

ICommandSource for RadPanelBarItem, using the same principle like RadMenuItem, since RadPanelBar is mostly used as a menu.

Update the ExpandMode property to allow zero, one or zero, one ,and multiple expanded items.

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.