I've been using Windows Seven ( ) from the early betas with no issues whatsoever. Except for the surprise of having to install DirectX9 and its updates manually (Win7 autoupdates drivers as they are released).
as a more general answer the only thing I couldn't get to work in win7 was Optical Drive Emulation apps, like PowerISO, and such. that was with the initial beta win7 version though, and there are win7 versions of all of them as far as I know now. other than that everythings worked flawlessly.
btw you're in for a treat, it's so much better than vista. no stability problems at all so far, and I've had it since the release candidate version