...what does Win7 have that XP can't do?
Hawk
Future-proofing.

Microsoft will discontinue support for XP in a few years. Better to get used to Win7 as soon as possible, rather than be forced into it at a later date.
Of course, Win7 also has optimised memory management (which makes the computing experience feel smoother and quicker - for example, when an application is minimised, the memory for that application is freed up). And there are several other features, some of which I'm sure have already been mentioned in the various Win7 threads on PCD, and all of which can be found with a quick bit of googling :)
My only negative about Win7? Lack of driver support for older hardware. Which, for me, equates to
1) having to use a custom driver for my audio (which means that I'm limited to enhanced stereo and a little bass, but nothing like the quality of sound I was accustomed to using the WinXP Creative driver with my 5.1 speakers and compatible sound card). I will need to buy an Audigy or above to rectify the situation.
2) not being able to use the VIVO capabilities of my 7800GTX, which means I can no longer watch TV on my PC or save VCR recordings into a digital format. I will need to buy a stand-alone TV card to rectify the situation.