In many ways, Oblivion doesn't care that you exist. The world will get along fine without you. I get to make up the story as I play along. In one game I'm a pack-rat alchemist. In the next, I'm a wannabe cartographer who wants to map the whole territory. In a third, I'm a Holy Knight who can't stand the sight of the Oblivion incursion. In a 4th, I only care about robbing graves. Oblivion was endlessly fascinating for me. Morrowind was the same, but with more interesting quest-lines.
That would be fine and dandy if your actions in the game, whether as a cartographer, alchemist, Holy Knight or grave robber actually had an impact in the game.
But I actually think you have that backwards: Oblivion cares too much that you exist, that's why you have a quest NPC GPS, enemies and items scaled to YOUR level, and an invasion that actually NEVER occurs except in one town (the gates wait for YOU to come and seal them).
I agree to some point, you don't need a whole lot of story in a game for it to be good. Diablo type games come to mind. I played Titan Quest recently and sure, there is a story but that is secondary to just "killing stuff", collecting loot and levelling.
But in a world (Tamriel) with such an abundant amount of lore (and most of it very interesting and well written), a story IS somewhat essential. But I digress - my main problem with Oblivion wasn't the story or lack of story but the dumbing down from Morrowind (see points above) which I loved.
Then there was the blandness of the world/NPC. Part of this is the change of scenery but I can deal with that (it still looked great, if a bit generic). But the simplified dialogue and poor voice acting was the final nail, hehe.
And I've said this before: I'm not expecting Oscar Award worthy voice acting, just that the actors in Oblivion truly sound as if they're performing for children and it's bad enough the dialogue is poor to begin with. Add to that the switch of actors from one sentence to the next (which I think was the biggest problem I had with voice acting) and really, it's hard to suspend reality no matter how great my imagination is.
Morrowind was just far more intriguing as a game world and THAT made me overlook the game's flaws. I also preferred reading the dialogue even if the dialogue system felt like an encyclopedia or database sometimes.
See? I just talked myself into not playing Oblivion again (*pats self on back*)