Don't get me wrong, Oblivion is a very good game, but it lacks the really distinct atmosphere that can make RPG's great.
First of all, the voice acting is all very well done, with one exception: It sounds as if Bethesda hired a very small group of people to cover all the characters in the game. Furthermore, these actors did almost nothing to change the characteristics of their voice from character to character, which leads to a grand total of maybe 10 distinct voices in the game to cover all the hundreds and hundreds of NPC's. This can greatly detract from the game's atmosphere, especially when there are two characters talking to one another who have the same voice, and at times the problem can be downright disappointing.
The main problem with Oblivion is that it lacks a distinct atmosphere, and much of the game reeks with being generic. There appears to be little thematic content, and after a while I got tired of all the "go find my stolen item" quests. Games like Thief and even KOTOR had a certain pizazz that made them enjoyable and at times thought provoking, but Oblivion does not deliver in this category. Also, the problems with voice acting contribute to this. Instead of giving the feeling that it's game world is teeming with unique characters, Oblivion gives the impression that there are five or six major characters and fifteen or twenty generic characters that are repeated.
The one other weak spot in Oblivion is the combat system. Melee attacks are limited to a slash, a strong slash, and, if you invest all your skill points in blade and/or blunt, a strong slash with the chance of stun. Thus, many battles are boring, though a certain degree of timing is required to swing the weapon at the right time. I cannot help but get the impression that Bethesda opted for quantity over quality in this game. Though for what it's worth, it's very polished and well done, those seeking a moody, artsy, or heavily atmospheric RPG should look elsewhere.