In particular, I'm hopeful that the radeon and intel FOSS drivers will improve (although I guess that nvidia said they will help with nouveau now:
http://arstechnica.com/information-technology/2013/09/nvidia...). I gather Valve has given significant feedback to all three vendors (
http://www.pcworld.com/article/2049369/amd-nvidia-ramp-up-li... and:
http://games.slashdot.org/story/12/08/31/1551230/valve-finds...), and that steam games have, perhaps, stressed the sort of hitherto rarely used code paths that would only show themselves to be broken or unreasonably slow when some compositing DM feature wouldn't work, for example when KDE 4 was famously broken on fglrx for a couple of years, and no one cared enough to fix the driver bug because who plays games on linux, right? Of course that's the closed driver but the same kinds of problems have happened for all the drivers for want of heavy use, and of course they won't be heavily used if they are way broken.
Please correct me if I'm wrong, but I think Wayland/Mir support must be added to any driver that will support them, and I believe it's not yet clear that that will be done. Intel has already refused a pull request for XMir support (http://slashdot.org/story/13/09/08/0431242/intel-rejects-sup...), and I think the proprietary drivers have no support for either of the new WM's, nor will will they publicly admit to having any plans to implement it.
KMS was added to the kernel five years ago this month, and the proprietary drivers still don't support it, although the intel driver does. Imagine a similar outcome for the new compositors, where the vendors don't implement support for a long time, or, worse, where there are factions comprised of certain combinations of driver/distro/WM... that would be bad.