I disagree. The primary failure of Oblivion as a game is that every quest is spoon-fed to the player. You're told where to go, what to do when you get there, and you're not allowed to deviate. The possible dialog choices are limited to those which force you down the game's canned pathways, the quest NPCs are made impossible to kill, and you can't remove quest items from inventory. All of which means that even quest "failures" are canned; other than with your own character's death, you can only fail the Main Quest at a couple of places.
Most of the quest lines have very simplistic, juvenile "story lines." The sub-quests are mostly "go to this dungeon, fight monsters, and bring me back X."
Mods add content, and they fix some of the glaring issues (like level-scaling, closed-cell cities, and bugginess), but they don't (and can't) do much to fix problems like limited voice actors, rigid quest-line choices, or lack of quest-line plot subtlety. It's quite true that one can reverse things like "Essential NPCs" and "quest items you can't lose," but one can't really mod out the plot linearity which makes those things necessary in the first place. (Just as an example: In Morrowind, you could accidentally kill off "essential characters," making the normal Main Quest line impossible to finish, but there still remained a "back door" method by which you could "save the world." It was a more complex game, with more possible interactions, and more varied outcomes.)
Oblivion is an excellent roleplaying sandbox, but it is actually a very poor quality game. There's very little mystery, very little chance to fail, and the only outcome of all the main quest lines is that you become the faction "boss."
I'm Commander Shepard, and this is the best reply in this thread.
Oblivion is, as others have said, how I envisioned the ultimate rpg. I love it. Been playing on and off since release, and all vanilla too; many characters, many variations. It just doesn't get old.... I love it! I agree, I don't like that there is very little chance to fail, as in, no chance to fail. I want to have the chance that I can royally screw up a quest.
When I get a quest, I want to read directions and make my way to where I think the location is. I don't want a marker. I'd rather have: " Go two miles east until you see the big pasture. Take a left at the fat cow in the corner...." Things like this. I enjoy doing the pilgrimage.
Dual-wielding.
Deleting spells.
When two enemies are standing side by side, and I take down one with a single arrow, the other guy should freak out. Instead they stand there, daydreaming.
More joinable factions. And more: being able to do work for those factions. I want to join the Legion. I want to have to stand there and guard and do my duty. I want to get paid for my work. And if I do well with one faction, perhaps a rival faction feels like they should take me out. Legion vs the DB. I realize I'm reaching on the last one...