Well, I guess I disagree with most of you.
I love Oblivion. I think it's heads and shoulders over Morrowind, and any of the previous Daggerfalls.
The faction quest lines are quite good. I liked the Fighters Guild and Mage Guild lines, and I really liked the Thieve's Guild line, and really really like the Dark Brotherhood line, which surprised me, because I'm not a 'dark' type person.
There are more places to find and explore than most people have the patience to do, in my opinion. It is more of a sandbox world, of course, why else have a construction set allowing you to completely rebuild the world?
Just my worthless two cents, it's beautiful beyond belief, and I enjoy just 'being' in the world, and I enjoyed the Guild quest lines immensely as well as the main quest line. Are there some things I would like improved? Sure, but nothing so earthshattering that I'd bad mouth the game publicly. My thanks to Bethesda for Oblivion, I still enjoy it after all this time.