Nothing major is wrong with XP. Several things are wrong with Win7. I do have Win7 at home, cause I got a new laptop a year ago and didn't have much choice (and even if I felt like replacing the OS with a copy of XP from elsewhere, I'd have to fight with all the hardware, which I'm sure wasn't tested on XP since it's not supposed to be supported any more).
But after getting Win7, I had to find replacements or other hacks for dang near every piece of the visible UI, before I could call it useable. Some of them were just designed to look and feel like XP, or, more accurately, like Win2k; some of them were actually better than XP/Win2k, it's not like I would claim either of those OSes were perfect. They were just entirely useable, which is more than I can say about the mess of a UI that Win7 provides.
So I'm keeping my XP at work until IT pries it out from under me, because I don't feel like dealing with the same issues again. (Plus, it's nice to have developers working on different OSes. After all, we sell to precisely the sort of people who are going to want our software to continue to work on XP and on Win2k and Win2k3 for all eternity.)