Skill Based System.

[Jakensama]

Oblivion proved that people will put up with repetitive gameplay that lacks any interesting storyline even in single player games, provided they keep having the carrot to twink out their character with more and more powerful spells and daedric armor..

I think Oblivions popularity is much like WoW in that it recognizes the appeal of a game to a large audience where you are continually improving your character.