Windows Vista biggest bang is a 3D graphics engine. However, this can only be utilized with powerful gaming graphics cards, lots of RAM and fast cpus. Other than that it's XP in a different wrapper.
And, IMHO, XP, based on NT technology which is far better that 95/98's FAT technology, is well worth the price of admission. It is more stable, faster, easier to use, more complete and compatable with all current programs.
As a note: NT 3.51 was the only true great OS because it was specified and tested by the US military, not Microsoft. Nothing against big blue, but a mil spec wins every time and for a very good reason.