The problem is, it doesn't get out of the way properly when you run a program that uses SDL to change resolution and go fullscreen. This is hard to explain but basically instead of seeing what the SDL program is drawing, I get a black screen, with a white line across the top and down the left side, which looks remarkably like the top corner of a window.
Why is this a problem? Well, guess what is an SDL program that tries to go fullscreen and change resolution?
Basically I can't play Doom unless I run a second X server (startx -- :1)*, change resolution manually, (xrandr -s 640x480), and then run the game. This is really annoying! I have to stop myself just idly typing "doom2 blahblah.wad -warp 1" into a terminal like I've been doing all the time for years.
I think it's awesome's fault because I've tried various other minimal tiling window managers such as ratpoison, wmii etc. and they all do what I expect. But they have other issues and I don't really want to change window managers.
* Not strictly necessary, but using xrandr under awesome causes awesome to reset all of your tag layouts back to normal, which is just as annoying, so a second X server running it is.