I think oblivion could have been a much better game if they just spent more time on it. It seems they spent all this time working on a brand new engine, state of the art graphics and the newest AI they didn't have time to make the actual game. In the end the brand new engine meant they couldn't include things like horse combat, crossbows, throwing weapons and the fun spells. Graphics were good though, but they were kinda dull in a lot of places. The game world was significantly smaller, ok, they claim its 50% larger, but that doesn't count when you move 100% faster and objects of interest are 150% less dense, you have less cities, less NPCs, less quests and less unique spaces (though there were some). And the new AI was chaotic, so they can't claim they fixed/removed the bugs and they only programmed in the AI's use for about 20 NPCs, and even then no where near the full potential of what they could do.
So for TES V, I hope they a) ditch the oblivion creature leveling system because no one likes it.

Use less out there technology and focus on what they can do so there is lots of game to be played rather than lots of bugs and missing features. c) Make the game dirty, ugly and realistic which was another thing which made the series different. I always want to throw up when I see final fantasy or WoW graphics, they just look so compassion and cartoony. d) extend the low level area of the game, why have chittin in MW if you can get iron or even steel immediately? and the low level, poor part of the game is damn fun. e) Make it so you can spend many many hours in 1 city or the forest without going somewhere else, maybe with minigames, or functional and interesting merchantism and hunting, it adds to the variety of a normal game and gives the possibility of themed players.