The most obvious thing you will notice about Oblivion is that it is not for everyone. The occasional RPG fan will pass.
Playing an RPG in first-person adds something to the quality of the overall experience and when done so nicely, and beautifully like here in Oblivion you can appreciate the detail in the hand models and weapons nicer. But the game takes time to get into and used to. Once you realise the travel can almost always be quick, that you will not find enemies often throughout the main world like in traditional MMO's then you can really start to see what is so good.
I am not going to talk a lot about what makes Oblivion so great because it has been said a million times before. But I am going to go into those oh so hard to find flaws that the game incorporates. The first thing to note is that it does freeze. Not just on a broken xbox but on all of them. About once or twice in 10hours of gameplay I would estimate.
Next as stated above is the lack of monsters to fight. Because you do not regenerate health fast and cannot take much damage anyways, you will not find many monsters to fight. When you go into places like caves, hideouts, and oblivion gates sure you will run into 5-10 bandits, or rats, or other things depending on your level. But don't expect to find the lone mobs to rush into and fight then move on quickly.
Lastly is the frustration of leveling. The system itself works great, but where it flaws is the actual amount of time required to reach those higher levels, newer mobs, quests, and spells. Unless you rush to level and hinder your character your looking at hours and hours and hours of playing time before reaching 10. This can be extra frustrating for those who like lots of adds(additional players).
Overall the game is great and still worth it for anyone looking for a RPG, to kill time, or generally find some nice eye candy to see what microsofts next-gen system is capable of.