It's not, people just like to hate on things because they are new.
Vista has much better memory management options, is much snappier than XP, has built-in index searching, shadow copies, UAC (a blessing not a curse like some narrow minded people choose to see it) and much better security, I've been running Vista for almost 2 years now without an anti-virus (just Windows Defender) and I haven't had a single viral problem.
Vista also has DX10 support which XP doesn't and will never support (Alky only plays demos and as far as I know, it's not even being developed anymore) and while game performance is indeed a little lower, SP1 did improve performance and the difference is only noticeable with a low-end card.
Vista has a lot of great features, it also has a bit of issues (like incompatibility, etc) but since SP1 it's a pretty hassle free system, at least for me, my workflow is much smoother on Vista, I only keep XP for legacy purposes (some very old games like HoMM3 or even older like DOS games have difficulty running properly on Vista).