There are a couple questions you might ask, actually:

1) what's new in Vista for end-users?  if I sat my {mom | dad | brother | neighbor} in front of a Vista desktop, what would they see as different?  Lots of changes here to explorer and apps (music and picture experiences, for example)

2) what's new in Vista for developers? Lots of new platform stuff.  Much of it (WinFX, for example) will be available on XP and 2003 as well, which is great news.  But there are some APIs that are unique to Vista (new stuff around inking, power management, p2p, etc)

3) what makes Vista a better OS than XP?  This is an all up question.  you'd certainly include the answers to #1 and #2 in here, but then you could also list a bunch of other feature areas: improvements to core OS for performance, stability and reliability (LUA, IE 7 low-rights mode), improvements for IT around imaging/deployment and manageability, and improvements for OEMs.

It would take many pages to answer quesiton #3 -- I will look around to see if there's anything on the public ms.com web site that tries to summarize it all.