In my experience, WinXP was slower than Win2000. And please do not cite "prefetcher", it makes ANY operating system slower in my experience, so I always disable it.
Most important, WinXP was MUCH, MUCH more buggy than Win2000. In Win2000 I haven't spotted a single bug, in WinXP I could name some 15-20 that I faced.
Most of the bugs were visual glitches, sometimes related to the theme engine though. The most annoying bug for me was that the start button would go pushed only after the menu apeared, and not when clicked. Another annoying bug was that the language panel would make the taskbar thick in Classic theme, so I would always take the internat.exe files from Win2000 and replace the layout indicator with it. It would reside in system tray.
I also disliked the artwork of WinXP, including the icon theme and how some visuals intended for Luna would still be pushed in Classic theme (such as the aforementioned language panel which was sized for Luna taskbar). The logon window in Classic theme would have weird window metrics and colors imitating Luna. Console window would somtimes disrespect theme settings. And so on and so on...
I admit, WinXP had some very useful functions compared to Win2000, such as system restore, ability to arrange icons on the desktop by a grid (this was possible on Win2000 via third-party software), Wi-Fi management, ClearType. But overall WinXP had much worse quality and visual consistency. It also was slower.