A lot of issues you probably won’t even detect in this modern age of self updating apps. You can’t run the newest version of IE on XP (not such a bad thing, nothing makes a version of IE look better than the next version), but Windows won’t try. Don’t know if current Firefox supports XP, but again it’s self updating so if it doesn’t it wouldn’t try. I know we’re dropping XP from the stuff I work on, finally. You’ll probably start running into more things dropping XP now, .Net4.5 doesn’t support anything before Vista, so anybody that’s jumping to that has to drop XP.
I was using XP64 for some time, however there is much more that will refuse to run there.
I have an 2 xp systems for most things and a win7 box for the new games and not much else.