The compositor works, I've tried it, but it slows down 2d rendering performance a lot, even for my fast r500 card. They're trying to resolve that now.
Gnome definitely needs more capability, and more customization available from the gui. Totem needs a decent smart streaming media buffer. Project Ridley needs to be in full swing to bring some important stuff (Like GTKGLEXT) under the umbrella of gtk. Cairo will soon get a scanline rasterizer, I think, which should close the performance gap with QT. Compiz is nice, but it has no graceful fail for computers without fast 3d, and it eats gpu resources in a fashion reminiscent of Windows Vista (although not quite that bad.)
Oh, and going with the compositor, metacity themes should become argb-visual-capable. The age of the jaggity 1-bit alpha round corners is long gone.
I'd also like it if gtk were a bit easier to work with, perhaps being event-based rather than having a main loop (all the nitty-gritty doesn't really need to be handled by the app developer, and it would make it less complicated to get started with an application.)