From time to time I like to take a look at new things, like e.g. Gnome3 and its gnome-shell.
My setup uses proprietary nvidia drivers and two seperate X screens, one for the LCD, one for the Samsung TV over HDMI (XBMC that is). This works just fine in KDE4, Gnome2 and anything else I've tried. But Gnome3 just blatantly fails to start with this setup. Last time I tried, it was Gnome 3.0.0 or something, but meanwhile there is Gnome 3.2.2 and still they didn't manage or refused to fix this issue.
Well, I'm not alone:
..and here the bug report: https://bugzilla.gnome.org/show_bug.cgi?id=648156