Mounted combat. No-brainer. There’s no reason why I can’t just look down sideways from my horse and click to swing a sword or shoot an arrow.
With this should probably come a restructuring of the horse-riding controls; e.g., “kick,” “rein in,” and “rein left/right,” with an option to rein “hard” left/right to get it to turn quickly, and there should be some AI built in to have it avoid obstacles on its own. It sucks to get hung up on a rock.
Supply and demand. People in City #1 ought to want certain goods from City #2 a lot and be willing to pay more than their nominal value for them. Right now the mercantile system kind of sucks.
A much better way to organize spells and access them quickly from the keyboard. (e.g., a key to quickly flip through all the spells in one particular school, or an option to “forget” spells that you don’t use any more.)
If you know how to cast a spell, you ought to be able to perform the target, touch, and self versions with different keys on the keyboard. And maybe an area-effect version centered about your character, too.
The haggling and disposition mini-game-things ought to just be built in to the conversation system.
Randomly generated mini-quests. This could be way easy. Some merchant has run out of X, do you have ten X’s to sell? Some dude has lost his Y, can you get him another one (bonus points if you find his)?
Lots more of the puzzle-solving, clue-tracking, and negotiating varieties of quests, and fewer of the dungeon-crawling variety. Leave those to the random ones.
Players should have to eat/sleep at least once a day or suffer a penalty in fatigue. I mean, there ought to be an incentive to do these things.
Different types of armor/weapons ought to behave in qualitatively different ways. e.g., glass would be absolutely stupid to make armor or battle axes out of, but I ought to be able to stab some ogre with a glass dagger and then break off the blade, dealing massive amounts of damage. Or set a guy in ebony or fur armor on fire.
The amount of damage my swords and arrows do should depend on where I hit a guy. And I don’t care what level he is, if I put a poisoned, enchanted arrow through his head, he should just drop dead.
While higher-leveled enemies ought to be introduced into the world gradually as the player levels up, there always ought to be a good deal of the lower-leveled enemies that the player can knock off easily.
I think there ought to be a few other adventurers in the world, doing the same things you are–maybe not the main quest, but they ought to be accomplishing things, completing side quests, and affecting the state of things in the world. This could make for really interesting quests where you would get to compare notes with/work alongside/fight against your competitors.
NPCs should have different disposition ratings for each topic they can talk about, and this should have a much more dramatic effect on what they tell you.
Some of the bandit groups/goblin tribes/city guards/etc ought to have their own objectives and go out in the world to accomplish them in a coordinated fashion.
Oh, and make all that run beautifully on my current one-and-a-half-year-old laptop.