1. Side quests
I understand that a game with the number of quests the previous TES games had will invevitably have some generic quests. By generic quests I mean quests like "gather X amount of Y items", "kill a specific bad guy residing at the end of a cave", "find item X in the deepest chamber of cave X", that sort of thing. Let me also note that Oblivion handled this better than Morrowind did, especially the Thieves Guild quests and the popular Dark Brotherhood questline (despite the fact I disliked the way the brootherhood was depicted).
However, I feel that there is so much room for improvement. At least the quests you do for the various guilds should all be spiced up with unpredictable twists to keep players on their toes. It doesn't always have to be a plot worthy of a Hitchcock film, but you can always do little changes to deviate from the same old boring stuff we've seen done in so many games before. Maybe when you go to find item X, you don't find it.You discover clues that it has already been taken by somebody else. Perhaps a long chain quest begins, perhaps you just follow some tracks to a hidden exit from the cave, that leads to a nearby camp site. Perhaps the grave robbers that already took the item are there, perhaps you have to follow other clues or return to your questgiver empty handed. Perhaps if you do return empty handed, the questgiver tells you how he has received an anonymous note asking for an exorbitant amount of money in exchange for the item, and sends you to investigate. (I'm not claiming this would be an awesome quest, I just wanted to show how easy it is to make a quest more interesting without much effort). You need to keep players guessing what will happen next.
2. Characters and Dialogue
This aspect, I feel, Morrowind handled better than Oblivion. I do understand that with the sheer number of NPCs means that quite a lot of them don't need to have deep personalities. However, I believe that this has traditionaly been one of TES series' weakest points. I would love to see at least some of the NPCs you more often socialise with have more proeminent and interesting personalities. I would also like to see dialogues both more detailed and with a wider variety of issues to discuss - it would help you understand the personality of each npc, what makes him/her tick. Voice acting is a nice addition, but what difference will it make if the NPCs are not interesting enough?
3. Main Antagonist
This is where Oblivion got it very, very wrong. I understand that Mehrunes Dagon is the Daedra Prince of Destruction, but he is also the Prince of Change, Revolution, Energy, and Ambition. As a main antagonist he was as one dimensional as they come. They probably tried to counter this by introducing Camoran, but honestly, he wasn't much more interesting either. In my opinion, making the main antagonist the personification of evil without any aparent explanation, is the best way to make the main quest uninteresting. Dagoth Ur - despite the horrible voice acting and the minimal dialogue you could engage in, was a much better "villain". Because he was complex.
Think of all the memorable villains in RPGs, none of them was just pure "evil". Remember Irenicus, or The Master? Those were what villains should be, not Mehrunes Dagon. This is one of the rare things about Skyrim I am a bit pesimistic about: Alduin sounds quite likely to end up as just another one dimensional all consuming evil. I hope Beth prove me wrong - they have a rich material to work with.
4. Ambiguous Moral Choices
I have always enjoyed the degree of freedom you had in TES. But all actions should have consequences, and since often you were not given the opportunity to make meaningful choices, most of your actions had no consequences. Players should feel like their actions matter, that they have an impact on the NPCs and the world, and that they have the option to make choices. I would definately not want to see choices reduced to "good path vs evil path" with an apropriate bar to show me how good or evil my character is. Games have evolved past that state of infancy hopefuly. Choices should be moraly ambiguous, they should get the player thinking of the positive and negative consequences of their actions, wich people and groups they will influence and in what way.
/rant over