(I'll copy and paste this into the next suggestion box, but I copy it here in case you're tempted to answer it.)
In Windows 7 I can't pin the "Microsoft Help Viewer" that comes with Visual Studio 2012 to the taskbar, and its jump list only contains a "Close" option (none of the other usual ones). I have never seen this behaviour from any other window.
In Windows 8 I can pin it.
So, what was wrong and what changed?