Speed, UI improvements/tweaks, security, etc. If you have vista, maybe do an upgrade, but it isn’t essential. If you have XP, I’d probably upgrade when it comes out.
There isn’t anything “wrong” with XP from an interface standpoint, but it has some security issues that they mostly tied up with Vista. MS is a good company and will throw tons of money at their current products to keep them up to date, but not so much their dated products like XP. XP support is over essentially, and I’d move on, unless your computer is a total POS< in which case you need a new computer ;).
If you have an old hard drive laying around, plug it in and throw Windows 7 on it, or even just check out the screenshots online, it’s a nice looking OS.
Overall I like vista more than XP, and in games it seems like my shots are hitting better than they were for XP (In fps games, and I don’t know why this is either, it’s just something I saw).