Lets start with morrowind. That game had amazing graphics at the time and you could explore and do what ever you wanted in an immersive world as well as follow a pretty good main quest.
But what lets down a lot of RPG's is the fact that the NPC's just stand in the same spot 24/7 like dummies repeating the same old poop.
Oblivion changes all that. The NPC's will have their own little lifes to worry about and their dialogue will change depending on what you have done in the game.
So lets recap. Oblivion will have amazing graphics, amazing interaction with other characters and hopefully some amazing quests that will keep you busy for months. Basically it takes the traditional RPG and throws in everything thats normally missed out like realistic lighting and weather effects, day/night cycles and NPC interaction as well as being highly moddable with a big community willing to do some amazing things like they did for morrowind.
But I think it will have to be played to be appreciated. Go buy it as soon as you can and find out for yourself why lots of other people are going crazy waiting for it.