Just wanted to get .02 in on Skyrim...
I also didn't care for the level scaling in Oblivion. Also, the dungeons and environments were repetitive. Don't even get me started on those Oblivion Gates, ugh.
They've fixed most of the biggest Oblivion problems though. The scaling problem is more or less fixed, the quests have more variety and the environments are more interesting. The character build system is probably one of the most fun parts: no major and minor skills. Just level whatever skills you want to. Bethesda set out to make the introduction part of Oblivion seem like the entire game and it worked.
As far as weaknesses go, there are a lot of bugs. Just look at the wiki of everything they've caught and are trying to fix so far. It's staggering. And, well, the combat is more or less as awkward as earlier games (but the critical hits do make things more fun). That being said the game has personality and you can tell they add little touches to the dialogue to so the npc's and enemies seem like real people. The next patch is coming out after Thanksgiving, it's apparently being tested now, so at least they'll be taking care of the worst things first. I'm going to say it's more or less a must-buy.