I've been a XFCE user since 2005, probably earlier, and every time I try a new Gnome or KDE desktop, I get back to XFCE. It works, it's light, the panel works just fine, Alt-Tab works as expected, Alt-` switches between windows of the same program (like the dozen terminals I keep opening all the time), you can have several virtual desktops and Ctrl-Alt-arrow to change between them. It supports icons on the desktop, icons on the menu, and takes very little memory.
The other desktop environment that I used extensively is Cinnamon. It's fast, it's pretty, everything works, and maybe someday I will take the time to build it under RedHat Enterprise, the distro I am forced to use because it's the company issued Thinkpad that I explicitly requested (otherwise I would have been issued a MacBook or the Air variant). But as XFCE is as good as Cinnamon, I have little incentive to install dozens and dozens of libraries and wrestle with make to, well, make it.