Mass Effect is a much more traditional RPG than Oblivion. The only thing that separates Mass Effect from other RPGs is the combat, which is much more like a tactical shooter like Rainbow Six. But your squad isn't that smart, they will often get stuck, face the wrong way, or just stand out in the open and die.
The conversation system is great, but it seems like they payed more attention to that than the other game play elements. Exploration isn't that interesting, pretty much all the planets you visit are barren and boring with the same crap just situated differently. The only locations that have any individuality are the ones that are part of the main quest.
Oblivion on the other hand is much like a sandbox game like San Andreas, in that there is much more freedom to do what you want. Even though Oblivion takes place in one little country, exploration is much more interesting because the landscape is far more varied. Even all of the dungeons and caves seemed to have been given individual attention, at least more attention than the side quest areas of Mass Effect.
I can't say that Mass Effect is a bad game, it's just not my kind of game, but I definitely enjoy Oblivion better.
