If I recall from the Win 7 blog, they explicitly improved memory management for Aero so that each process didn't need to keep its own copy. So memory usage is much closer to XP than Vista for 7 (still more, of course, but that's to be expected).
There's a lot of little annoying things that changed from XP to Vista though, that were improved with 7. It doesn't even seem like an upgrade to Vista, but 7 feels a lot smoother (and the new taskbar is nice). Other things (though I believe a lot was addressed in Vista's service pack) like UAC were more poorly done as well.
The new taskbar in W7 is HORRIBLE for multitasking. I usuallt have 3-6 windows of my browser open. It takes FOREVER to switch between them when it took a single mouse click in vista...