Vista is more secure and has a better firewall. Easier to set up connections and the search facility for files and folder is much better. Tabs on taskbar are 'live' so that if you have a scanner or something similar running, you just need to hover the mouse pointer over it and it shows a miniature live page. The eye candy, of course, snipping tool and a lot of other little things I forget..
My main desktop is XP and will be until it expires. A while ago I bought a reconditioned laptop with Vista home premium. CPU a tiny 1.7 GHz with 1 GB ram. Vista worked well under these conditions but after upgrading the laptop to 2 GB ram there was a slight improvement in speed.
I find that I use the laptop more and more as Vista is definitely a step foreward as an operating system. Unfortunately, though,with rumors of a Windows 7 OS not far away then maybe businesses will be better off waiting before upgrading. As for home users and possibly gamers, and after SP1 then Directx 10 games might be supported more by the game producers. If that happens then I fear that XP will be down and out.
Credit to XP though, was the best OS by Microsoft up to now.