Oblivion's graphics don't make up for an altogether weak and p*ss easy experience.
Oblivion lacks many things which RPGs need: balance, immersion, variety and a good story. We'll start with balance. Although Oblivion's class system allows you to pick and choose what you want from each class, there is little to recommend in the way of any other skills than stealth and weapons, since that's pretty much all you need to beat the game. The point of having multiple "roles" in an RPG is to create balanced characters that have to play off each other in order to work, a concept that Bethesda has failed entirely at this time around.
The next thing that Oblivion fails at doing is immersing you in the world. This can be done about a million different ways, one of them is an challenging and fun combat system, so lets break down Oblivion's: melee consists of pushing in the trigger while your guy swings his weapon around in front of him until everything in front of you is dead. Ranged is slightly more interesting in that you have to aim, but other than that is pretty much the same. Magic consists of equipping your best spell and hitting the bumper until all the Sh*t on screen is on fire. The final way to immerse yourself in combat is with stealth, but since a single stealth arrow will kill an enemy and once you've fired they know exactly where you are, the stealth system becomes either very dull from all the one-hit kills, or terrible because when you can't kill you are now being charged by every enemy in the surrounding few miles. In addition to the lack of immersion in the combat, characters are 1-dimension people who all appear to sound startlingly similar to one another. Its almost as if Bethesda had only about 4 voice actors because it cost so much for Patrick Stewart to be in the game for 10 f*ckng seconds. The final immersion problem is that Oblivion's world is massive, which would play to its advantage if there was anything unique or creative in its landscape and if you couldn't skip any traveling by using the f*cking teleport button.
One of the largest problems with Oblivion is a lack of variety. The combat is the same throughout the game, and since the game is adjusted to your own level, everything is stuck at the difficulty of p*ss easy (even with the level adjusted). In addition, Oblivion seems to have abandoned the creative and atypical mud-fest that was Morrowind in favor of the fantasy world that every other f*cking game takes place in. Despite the graphics being pretty at a glance, the game fails to deviate at all from green rolling hills, trees and ruins. Once you've been to one f*cking cave you've seen them all. This normally isn't even too bad but since Oblivion is a game you're meant to spend upwards of 100 hours with it becomes monotony at its best and f*cking depressingly boring at its worst.
Finally we get to the bread and butter of a game, which can save it from utter destruction by awful game design, and that is the story (example of such a rescue would be Radical Dreamers). Unfortunately, Oblivion's story is fairly predictable and boring, as the rest of the game is.
Overall, Oblivion is an enjoyable game for a little while, but it is beyond me how everyone has managed to stick to this game for so long unless they have the memory of a goldfish so they can't remember that every cave of goblins is going to be pretty much identical.