Combat is somewhat similar to Oblivion, but tuned for 3rd person, and without having a shield. You've got a quick attack (left mouse), power attack (right mouse) and parry/block (E). At first it seems like you're randomly clicking, but you actually have to time it, and pay attention to what your opponents are doing (and if they knock you down, you're pretty much screwed).
NPCs that occasionally accompany you have been invulnerable so far, and range from not helpful to helpful (they haven't gotten in the way yet, like some Oblivion companion mods). You have no control over companions, either (some mods gave you some control).
And the graphics are very, very pretty.
That's where the similarities to Oblivion end. Maps and moving through are lot like Baldurs Gate and BG2, actually, with an occasional on-rails section (this is rare, and happens when you've got something trying EAT you).
While you play as Geralt, you have lots of customization options with four skill trees (think Diablo 2 on steroids). You can also acquire bonus attributes and upgrades by finding hidden places, specific books and performing dialog actions.
Most of your gear comes from the crafting system - it's usually better to by the recipes and materials offer by a merchant or workshop than the equipment they sell - you get better stuff that way.
Witcher 2 is more what you'd expect from 90's era Bioware given modern technology than something you'd get from Bethesda; Oblivion's mostly a different style of game. Oh, and don't forget the game's ale and whores - those are very important