One of the[If not the best] RPGs ever made
There are main quest to do, guilds to join but you could just ignore them and do what you want. This is what a real RPG should be.
There are 2 gameplays in Oblivion. Either the First Person or the third person view. In the third person view, you can't really target enemies well and when you see your character jump, its kind of strange. Although the third person view sucks the first person view is awesome.
Doing what you want to do is really fun. You can be a vampire, arena galdiator, a mage, raid dungeons and go treasure hunting, buy a house, buy horses or even steal them.
Leveling in Oblivion is one of the most addicting aspects of the game. Unlike other games, leveling is determined on the major skills you level. although it can get quite tiring using the same skill over and over again. The game difficulty will always be matching your level which means the enemies will always be near your level.
The music usually fits the game or place you are in. Although there arn't many of them.
Like Morrowind, you can create your own spells. There are tons of spells in this game. You can create a fireball with properties of ice and lightning rolled into one or even fly!
Oblivion is seriously a nice game to play. Try playing it even if you don't like RPGs.