I've had many PC save files where I never even beat the Main Quest. Either my file got lost with time, or saved up for all eternity and never bothered with again. It was only on one of my 360 characters that I finally got around to beating the Main Quest, and that was about a hundred hours after I'd done other things.
The thing about Oblivion is, for all its glitchiness and bad leveling ideas, something it does really well is good side-quests. It's not like the MMORPG side-quests where you have to kill x number of monster y to collect randomly-dropped item z, or find item x in convoluted dungeon t. The side-quests in Oblivion all tell an interesting, unique, mini-story. And they involve doing interesting, unique things. Finding out why everybody in a town is invisible. Making the sky rain dogs. Locking up a corrupt imperial guard. Quests are more fun when they're not just about the killing. They build atmosphere by showing that the society has problems of its own.