I finally got a laptop that could run Oblivion a year ago, and I've played it off and on since. After seeing the complaints here following the game's release, my expectations were low, and the game was a pleasant surprise. The interface was useable after a few days, the simplified armor wasn't all that limiting, and the writing was far better than I dared to hope, with more lore references than I had come to expect from the forum critics.
I know that many people love the graphical improvements and the combat engine in Oblivion, but if there is any one feature that stands out to me as a huge improvement over Morrowind, it would have to be the creature animations. I hate the generic nature of the Cyrodiil wildlife, but I love that they actually have different attack styles and tactics. In Morrowind, they all just run at you and strike (and Scribs paralyze you). In Oblivion, some enemies kick, some jump, some dodge or even disappear. It makes encounters much more interesting.
That said, I've found myself playing Oblivion less and less. The reason is that, even with mods like TIE, Oblivion feels generic. There is nothing in the scenery that takes my breath away, nothing in the city designs that make me love or hate the place or imagine any life there beyond what I see. The colors of the rooftops change, and that's about it. The general attitude of every city feels the same, with the possible exception of Bravil, which is a corrupt slum. The leaves on the trees and the colors of the grass change a bit as you travel, but again, that's pretty much it. Everywhere you go, you see the same types of ruins (that appear to serve no original purpose) and the same types of wildlife and people wearing the same types of clothing. It's pretty, but in only a few visits, I feel like I've seen everything there is to see.
I will continue to play Oblivion, because I need to finish my character's story, but I will continue to replay Morrowind because I can always come up with new stories there. My imagination is enlivened by the uniqueness of the House cultures and Ashlander tribes, the different environments and the wildness of the ruins that come from lost civilizations.
From what I see so far of Skyrim, my hopes are raised. It will not be a Morrowind clone, and it would be bad if it were. However, if the holds have different personalities, different value systems, and different architecture (as hinted in the limited screenshots thus far); if the creatures behave with at least as much uniqueness as they did in Oblivion; if the landscape is as varied as the teaser map would indicate; and if the ruins have a history to them as the stories of the dragon temples seem to hint at, then I expect that I will be satisfied. That type of depth is what I love about this series, the ability to see what's on my screen and use that to stimulate my imagination until an entire world of politics, religion, history, and ecology unfolds in front of me. Anything beyond that - graphical power, quests, etc - is important, but it's more or less icing on the cake.