Especiall for wide lists it would be very helpful to be able to zoom in and out of a RadGridView, in order to see more or less columns. 

Also see: http://www.telerik.com/forums/scaling-of-radgridview-content

When you define a style targeted at GridViewCell with a color without transparency, horizontal grid lines are hidden.

See this thread for more info:
http://www.telerik.com/forums/alternaterowbackground-doesn't-merged-with-rowstyle

It's  really common scenario, because a standard WPF DataGrid works like this.

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.

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

The same works fine with the old Nested mode.

RadGridView's FooterRow is still part of the visual tree, even though its Visibility is set Collapsed.