1. bigger screen.
2. different interface and controls (I would consider it much better). To me the best controls ever. I like tactile feedback, real buttons, not fingerprinting up a screen, etc. It has that...but it also has touch integrated for fast navigation. win win!
3. wifi. you can sync wirelessly...if you use a dock you can more or less function as a portable DVR (up to DVD rez, component out) which will update itself automatically and wirelessly from content on a central computer. This is also a nice feature for those who want to leave it in a car.
those are the biggest reasons for me. SQ is extremely good (still has a tiny bit of HDD spinup you can hear with IEMs and a quiet room) , but I haven't heard a 6G ipod to compare.
Basically for a combined DAP and video player it is an awesome solution IMO. A screen big enough to be useful with enough storage to be useful, in a small enough size to be really portable and slip in a pocket.
Personally I would have wanted an 80GB iPod Touch instead, fingerprints be damned. But there is nothing near that capacity, and even at 1/5th the storage the 16GB costs 60% more....