I like card stacks — navigating both the list of running applications, and the list of open Safari tabs, is pretty much a chore on iOS, and Palm has a big advantage there by treating each web page as a separate open app (um, I think that’s how it works?). Just Type, or Quicksilver for mobile, is very promising if the API is well structured. And Exhibition, which lets apps display ambient information when the phone is docked, is something iPhone users have been dreaming of for years.
Nothing huge so far, but very nice refinements; I can’t wait to see what devices HP proposes to run this OS on. (Is there any chance at all we could be spared the inevitable iPad clone?)