Upgrading from Win 9x/ME to XP was a must for everyone, because it was moving from the awful 9x kernel to the much more stable and secure NT kernel.
This made a massive improvement in stability. Programs could crash without Windows actually crashing - this was a totally new thing.
But what does Vista have to offer for most users?
Improved security - this can be achieved with 3rd party software anyway (firewall, antivirus, antispyware).
New software - IE7, WMP11, Windows Defender, Windows Search, these can all be downloaded and installed on XP. There are new things such as a new email client and calendar, but this is no use to anyway who uses Office Outlook or alternatives anyway. The sidebar can also be replicated by using Yahoo Widgets or similiar software.
Fancy graphics - When I first moved to XP, I had a 550 MHz processor, and 128 MB ram. This was above their minimum requirements, but ran so slowly, and therefore I didn't use the 'XP theme' for years until I got a new PC. I will do the same with Vista, turn off Aero etc. until I have a machine well above their recommended requirements, because I wouldn't sacrifice performance over graphics.
I've only noticed one feature I would like in XP from Vista and that is User Account Control, as I liked that in Linux.