If I personally could change one thing, it would be - as lots of others have said - to give users a choice of the Windows 7 UI, or Metro (or Windows 8 as they call it now). But I thought Windows 7 should have offered users a choice of the Vista Start Menu as well.
I have been a consistent Windows user since around Windows 2.0 back in 1987. Over the years on my main desktop rig, I used Windows 3.0, 3.1, 3.1.1, 95, 98, 2000, XP, Vista, and now 7. Thus far the only real version that I skipped was Windows Me, but Windows 8 might be my second one to ignore. I used the two releases for several days and I couldn't handle the UI changes. You guys can say that they don't matter, or that I'm an idiot, but I still use Linux (the latest kernel) with fvwm95, I'm running Window 7 with "VistaStartMenu" (a program that makes the start menu behave like Win2k and Vista). I like my keyboard shortcuts to work like they always have and I don't want to change unless I absolutely have to. Call me old or set in my ways (I am both) but I'm not going to wrap my brain around a new OS UI any more than I'd change my keyboard from QWERTY to Dworak despite Dworak keyboards supposedly being more efficient.