I've played Oblivion for many, many hours, and I still haven't discovered many of its features and secrets. The game is filled with large towns, dark dungeons, ruined fortresses and caves; all of these places are unique and usually have a higher purpose, such as quests and hidden treasures. In addition, pretty much every single dungeon in the game is relatively easy to enter and exit, which means that the player doesn't have to search for a way out for hours, which was a big problem in the early Elder Scrolls games.
The game is very flexible - there are 10 playable race with different strengths and weaknesses. Shortly after choosing a race, the player is presented with a range of classes to choose from, as well as an option to create a custom class. Unlike in other RPG games, the classes aren't simply Warrior, Thief and Mage. Oblivion allows the players to create their own, perfect profession. You want to be a fighter who also know how to poison or burn an enemy? No problem. Or do you want to be able to sneak behind an opponent, just so you can cut him down with your two handed sword? Nothing stops you from doing that. No matter what you do, you improve in some way. Running and swimming increase your athletic abilities, while fighting and hunting increase combat-oriented skills.
The graphics and music create a wonderful atmosphere. Sometimes I simply want to stop fighting/doing quests/hunting/trading/talking/making potions/enchanting my armour/swimming/(...) just so I can sit down and look at the beautiful sky, while I listen to one of Oblivion's relaxing songs.
Unfortunately, the number of songs is very limited, which means that you'll probably grow tired of the soundtrack after a few days of playing Oblivion.
The storyline is arguably the worst thing about Oblivion. It's unoriginal and very basic - you're the chosen one and you must save the world from the upcoming danger. None of the characters appears to be significantly different from other people - literally. All characters look the same and sound the same. All of the NPCs in the game(reminder: there's roughly a thousand of them) were voice acted by less than 20 different people. This means that you're going to hear the same voices over and over again, no matter where you go. Even some of the main characters sound nothing different from a random soldier, or a common beggar.
The player can try to make friends and enemies, which sounds really great. However, it gets boring after a short period of time and it doesn't offer any real advantages. The number of dialogues is very limited, which prevents any kind of character development - sometimes you HAVE to choose certain dialogue options which you disagree with!(eg. agreeing to do something that doesn't interest you) In addition, none of the NPCs(except for one) have a unique personality which separates them from the rest. The morality system makes absolutely no sense - your best friend will kill you for taking his bucket, while the guards will cut you to pieces for stealing a potato, 5 minutes after you saved the world from danger.
Even though there's a lot of things to do in Oblivion, there is almost no variety - you get to face the same kinds of enemies in almost every single place. In addition, there are barely any types of weapon and armour - you get to use the same sword for 10-20 hours of the game, because you can't find anything better unless you level up. A lot.
In conclusion, Oblivion is an entertaining game with many features which cannot be found in any other game. It will provide you with many hours of pure fun and excitement. On the other hand, it fails as an RPG game because of its lack of choice and almost no variety when it comes to NPCs, enemies and item types.