XP is nothing more than an upgraded 2k but there are some pretty good improvements in XP over 2k.
- Restore points
- ClearType fonts
- UPnP
- Improved compatibility mode for 16-bit apps
- Support for parallel DLL's, the first Windows to effectively end the DLL Hell.
- Better support for PnP devices such as scanners and cameras.
- Upgraded kernel with many small improvements such as improved RAM handling, improved multithreading, etc.
Hmh, these at least cover the most obvious ones. So yeah, XP is better than 2k and if you don't like the XP theme, you can disable it.

But there's no reason to upgrade if you're happy with W2k. Windows 9x, ME, NT3 and NT4 are different stories though because these are obsolete and no longer supported.