From trying 7 and my friend using it day to day, ive summarised:

It comes with an ability to "KDE 4.1" your tasbar, to make it shiney and harder to use.
It is (aparantly) slightly less demanding than vista, though if your bothered about the tiny % you'll stick with xp.
It has the libraries feature, which is like normal folders aka music, pictures, etc, but all in one place.

Other than that i couldnt see any real change from vista so for now at least ill be sticking with vista, and even if its £100 if you pre-order rather than £lol, thtas still £100 when you could "aqquire" it for free, or for £15 through an educational/NHS license.