I don't see the fuss about the UAC, once you have you pc setup you'll hardly even see it anymore.
What is more important from my point is not being logged in with a user of the administrator group all the time. Activate the Administrator and change your normal one to the user group.
xp ran smoothly on my core 2 duo...just perfect not a single problem i could recall...then i got quad core with vista...oh that headache("windows explorer has stopped working and will now restart"- every day same old message, + for some reason i found that awfully lot of programs i used on xp, didn't work anymore.), then i put on 7....weeee, like being back on xp again, good fast...all working again, with a lot more eyecandy though
Try Ubuntu 9 or OS 10.5 to see what Vista could have been.
XP has always been a far better choice than Vista for most sites, far lower machine specs for good performance, runs all the software and it is supported by MS for a while yet, so why change what works well ?
MS seem to have finally got things sorted with Windows 7, and it seems to actually work out of the box.
I'm running 3 test systems ( 3 Ghz Pentium, iMac Core 2, Dell Core 2 ) and it outperforms any previous version of Windoze on all of them.
7 will be worth updating to but I suspect only Home Premium or Enterprise ( or wotever ) will be worth bothering with.
Most people require a basic system which actually works, doesn't cause agravation, and doesn't require a completely pointless systems upgrade just to run the operating system.
Vista has been flakey as hell on a completely new machine.. just got progressively more unstable. I ended up completely reinstalling the OS and it was fine for about 3 to 4 months and now I get the occasional BOD and spontaneous restarts. There are no dodgy programmes, no Virtual Machines etc installed... nothing but WHQL certified drivers yet it's still unstable.
All that and it makes a reasonably spec'd machine feel slugish.
I never had any such issues with XP unless I started installing beta drivers.
I've got W7 as a dual boot. First thing I noticed was how much quicker it was than Vista. No unexplained "thinking about it" pauses before getting on with opening an application or installing a package. On the downside, I have already experienced my first crash on it, (so I am now entertaining the possibility that I have a very intermittent hardware issue), but either way it's more stable and quicker then Vista ever was. So much so, that I'm giving serious consideration to buying it when it's released and that would be a first for me !!
XP - works but kinda old and not that good looking anymore
Vista - Good looks, good performance (can't understand people who constantly blame Vista, mostly its their own stupidness). There is even service pack 2 available for Vista.
Windows 7 - Very good
Everything of mine runs no problem on vista. 
Just got to have a little more knowledge
(Had to do quite a bit of tweaking here and there... but hey, I have the time. ^_^)