Buy. This. Game.
Plot
There are several plots in the game. Each guild has a specific, albeit somewhat weak, story (Necromancers doing nasty things to the Mage's Guild, bad stuff going down in the Dark Brotherhood leading sect, etc.) and then there is main quest. Nothing too special in the fantasy genre, but remember this is a video game, not a JRR Tolkien novel. For a game, it's very interesting and the characters are well voice acted. Plus, it gives you a wonderful excuse to go hack up demons in an otherworldly plane. 5/5 overall
Gameplay
Oblivion on the Xbox 360 has a simple to grasp, good control scheme. It took me a while to master the blocking mechanic, but that's ancient history now.
The aspect of Oblivion that I think is the best is how it works with so many play styles. I am actually not all that fond of RPGs (I prefer RTS games, and am pretty darn good at them), but the freedom of Oblivion has allowed me to go about things with an Age of Empires, tactician mentality (attack, retreat, defend, attack, retreat, defend, move in for the kill), succeed and have a blast. 5/5
Music
The music of Oblivion was composed by Jeremy Soule. It is a sweeping, epic score, similar to some of Soule's previous work on Guild Wars and Morrowind. It's good, very good. There's a lot more here than there was in Morrowind, which looped after about 5 minutes, tops. Oblivion goes on for at least 15 minutes without looping. Not to say that that's an incredibly good thing, but what do you expect for a 100+ hour game? The music is great, even if it is repetitive. 5/5
Graphics and Sound
This game is the best looking game I have ever seen or played. Even now, two year later, its vibrant world continues to outshine the majority of stuff on the market. However, the realism should be taken up a notch in the sequel, if only in the details (if you whack a table with a claymore, you should see a dent). The sound is also wonderful, with great voice acting from the likes of Patrick Stewart and Sean Bean. The sound effects are superb. 5/5