my issue with vista is that there aren't any compelling reasons to use it other than for 4gb ram.
let's look at the positive points:
1) dx10
this won't matter for a year or two. even then it will matter to a very small community
2) aero
it's sleek, but a theme is not a reason to upgrade OS.
the thing i hate is that you only get one color choice for taskbar: black. it's too dark and makes icons hard to see because of lack of contrast. they have a whole nice interface for changing the color of your window (which only slightly affects the taskbar) but only 1 taskbar color. users shouldn't have to use a hack to change just that.
3) security
everything being sandboxed makes a system more stable. uac does add more security despite it's annoyances. but xp is secure and stable enough.
when xp was advertised as more stable it was needed since win95/98/me did crash and cause problems. but xp is good enough. the more that vista offers is nice but not compelling enough to upgrade for. when's the last time you had a bsod of infection that vista would have prevented?
4) superfetch/readyboost
this is nice but again, not compelling. saving 5 seconds when you open an application isn't reason enough to use a new OS. loading time is such a small aspect of computing to improve on. average people aren't opening apps all the time.
the area where superfetching would be more useful is gaming. that's a situation where loading is annoyingly long and superfetching the next level/map/area would be nice.
but saving a few seconds off internet browser or office or email reader isn't stunning.
the big negative is that everything takes more clicks to do in vista. they added more menus and dialog boxes and warnings, that just basically mean more clicks than in xp.
this is the same problem in office 2007. having a fully customizable toolbar in previous office versions lets any command be accessed with 1 click. but now in 2007 you have to always click on a ribbon before getting the command. heaven forbid two commands you want are on different ribbons, and you'll have to click back and forth between ribbons all the time.