Probably the biggest wow factor for me is the fact that they've managed to make the game look even more kickass with the same engine. The gameplay changes look pretty neat too...and I like the new lineup of baddies to take down. More diverse.
I've been playing Fallout 3, then I put the Oblivion disc in the drive by accident but didn't realise I was playing Oblivion until about an hour in because it's the same damn game!
Our opinions may be different, but you have to admit the gameplay, the NPCs, the environment (list goes on) are just similar between the two games. I think they used the same voice actors even.