You have to remember, though, that Windows 7 uses Vista as a base. It's not 'entirely' new from the ground up.
With the RAM issue, Windows 7 uses 'round the same amount of RAM that Vista does, though they both use the RAM in a different way than XP or other operating systems before it (
http://en.wikipedia.org/wiki/Superfetch#SuperFetch)
Still, Windows 7 does have a decent improvement in performance, but I didn't really notice all that much on my current PC (2GB RAM, 2.8GHz Athlon X2, 8800GT, etc.), though there was a huge improvement on an older PC running XP.
Also, the new Taskbar is sexy imho.