All DragVisualProviders for the RadListBox, as described in http://www.telerik.com/help/silverlight/radlistbox-features-dragdrop.html, show as a bare minimum a horizontal line that clearly shows where the drop within the list will occur.

Unfortunately the RadGridView does not even have that out of the box for its DragVisualProviders, so dragging elements within a RadGridView provides no indication at all to where the drop will happen! This seems like a very basic feature the RadGridView should have to support dragging/dropping within it (which also seems like a very common functionality).

It would be very nice to have a "transpose" functionality in RadGridView. There are ways to transpose data instead of GridView, but this way it would be difficult to configure bindings, styles and other parameters. PivotGrid isn't suitable in this scenario because of lack of editing.

There is an example of what is needed: https://rotatedwpfdatagrid.codeplex.com/. I was successfully using this example with a standard WPF DataGrid, but I don't know how to accomplish this in RadGridView.

There are already a few threads about this feature:
http://www.telerik.com/forums/transposing-columns-and-rows
http://www.telerik.com/forums/radgridview-can-be-transposed
http://www.telerik.com/forums/displaying-grid-data-in-column-row-format
http://www.telerik.com/forums/reverse-column-and-row-in-the-grid

Thank you!

It's so strange that Telerik's components don't support such basic mechanics of WPF...

А new virtualization mechanism for ColumnGroups will be introduced with Q2 2015. A property EnableColumnGroupsVirtualization is exposed with default value True for enabled virtualization. Setting it to False will enable the old logic.

gridviewdatacolumn distinct filter does not respect culture when displaying dates.

All the other display do display the correct format

The problem should be resolved with lib version 2015.3.1019.

The reason for the performance issues is the size of the ItemsSource for the combo column and the internal lookup logic.

A new IsLightweightModeEnabled property of GridViewComboBoxColumn will be introduced with R2 2016. Its default value is False. 
When set to True, the new lightened lookup logic is triggered, thus the performance of the column is significantly increased.

The issue seems to be a bug with UI Row Virtualization since once I set it to False, it did not occur any more. You can also refer to the documentation on UI Virtualization: http://docs.telerik.com/devtools/wpf/controls/radgridview/features/ui-virtualization.html