Oblivion's main appeal that made it look like a better game it really was, was certainly Jeremy Soule's music. Same goes for Skyrim. Great music can really enhance the atmosphere, until you actually discover that it's all disguise, and that the game-world is blunt, empty, and lifeless in reality. Another major appeal of Oblivion (and Skyrim, for that matter) was, that you had seemingly freedom to to whatever you want in the game-world, except, that there was virtually no different ways to play your character out. If we take different weapon and spell skills out of the pool (press a key to shoot arrow, fireball, swing a sword, or dagger), there's no much different ways to roleplay your character in the game world - it all ends pretty much the same. However, if you look it not as a role-playing game, but an action game, then it's a decent one, I suppose (compared to the other action games available). Still repetitive with quite shallow game-world, though.
Skyrim and Oblivion are games that you'd really want to like, since you wish them to offer an experience they don't, when you start a new game, create your character, and hop into the game-world. It doesn't take long to realize, though, that these games are really repetitive, with dull, empty, and nonreactive game-worlds, with only so much (next to none) ways to role-play your character out.
The biggest choices you can make are probably, for instance, whether you join Stormcloaks or Imperial Legion (in Skyrim) - that's a choice which has an effect. A simple choice, but a choice nevertheless - now that's a rare case. Whereas, the most choices you make, for instance, are to pick from 4 different dialogue options, which all lead to the same ending, despite of what you just chose - that's the typical case. That just doesn't cut it. It's unsatisfying experience.
Replay value suffers, because even if the game world in size is quite big, all characters play out too much the same, really. It's not going to make a big difference whether you swing a sword or an axe, or whether you pick a path of a spellcaster, even; and definitely not whether you play an orc or a nord. Race is only minor statistical factor, hardly taken into the account on any other matter. Few times per play-through someone might be bright and remind you that: "Oh, look, an Orc! Far from home, eh?", or something similar. Once again, that just doesn't cut it. It's just pretty much the same gaming experience over and over, with varying loot.