Voted for sidequests, as I think that's second, but by far the boring and slow combat needs to be fixed first, for me to like it. It doesn't need to become insane like DmC or anything, but the combat is severely lacking, making any exploring much less fun.
More factions with longer and better stories. Having only four (I guess six, but Stormcloaks and Imperials are basically the same but with a different paint job) in the main game and then adding one in the expansion is really disappointing. Morrowind had nine factions (twelve if you count the vampire bloodlines but their quest lines are nearly identical) that all had longer and more interesting stories than what Skyrim had. The early missions actually made you feel like a grunt who needed to rise up through the ranks before you got to serious stuff. Immersion is the name of the game, people.
Oh, and bring back the skills/spells that have been taken out over the generations. I would like to have some very stern words with the jackass who took acrobatics out of the game that's full of mountains.