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.)
if gnome devs finally decided to make a big change, something big is brewing for gtk as well. that's what i suspect anyway.
probably gtk3 is likely to be announced within a few months.
edit: actually gtk3 is already underway : http://live.gnome.org/GTK%2B/3.0/Tasks