I upgraded a (virtual) Windows 7 machine from WLM 2009 to Live Messenger 2011.
It had no issues on 2009, but on 2011 the titlebar of the contact window doesn't work. The minimize/maximize/close buttons still light up when I hover over them with the mouse, but they don't respond when I click on them. I can't drag the window either. The window is also missing the 'Windows Live Messenger' title, and the frame around the window. The same issue when I switch to social mode.
Broken Messenger on the left, regular Messenger on the right. Both are running in their own Windows 7 VM, without aero (not supported by VirtualBox).
I can still move the window around using keyboard (alt+space > move > arrow keys). Everything below the titlebar is still fully functional too. The sign-in and conversation windows don't have any problem at all.
Using:
Windows 7 Home Premium (inside VirtualBox 3.2.12
Windows Live Messenger 2011 (15.4.3508.1109)
Except for Windows Update the system is practically, virtually the same as the other system in the screenshot.
No Messenger patches or add-ons, not even Plus!