You played Oblivion unmodded and liked it? Alright, let me ask you, what was your favorite part of oblvion? The completely fucked dialogue assignment? The terrible voice acting? The horrible dialogue? The lack of choice and consequence? The awful skins? The poor writing? The shitty main story? Or maybe it was the monotonous dungeon crawls? Or maybe even the blatantly bad animations?
The only thing that was good were some of the guild quests, like the dark brotherhood. The only thing that is different in fallout 3, really, are the guns, no more copy paste dungeons, and the fact that it is just a little bit better than oblivion, but not by much.
If you want another view of oblivion you could go watch the Zero Punctuation review on it. Just remember, replace the world oblivion with fallout 3 and you already have your review.