My biggest problem is the lack of fluidity. When your character swings a sword, he swings right to left, then the animation switches the blade in his hand and he swings left to right, there is no continuity to his motions. He could be swinging from high right to low left, then arc upwards and swing from high left to low right in a continuous motion. Why is the character's footwork disconnected from his swinging? Why is there no thrusting? No parrying with dual weapons? Why can't I thrust with my shield raised, the Romans built an empire by thrusting through the gaps of their own shield wall (maybe a counter attack perk, that executes a quick thrust or slash if I attack right after I blocked an opponent's attack)? Why isn't there a 180 button so I can change direction quickly if I overshoot an opponent? Why can't I sprint left and right to dodge attacks (really just need a quick sidestep)? Maybe I've been spoiled by the Witcher 2 and Assassin's Creed, but in those games your avatar feels like a swordsman, in Skyrim he feels like a lumberjack.
There's also a lack of responsiveness to the controls, in particular, coming out of a crouch (I often have to press the
alt key several times, to get my character to uncrouch), or sprinting (when I want to charge I often have to press the spacebar several times to get him to run, which can be a problem with shield charges). I also don't like the hold the button down mechanic for a power attack, it should be single button tap, so that you have a faster reaction time with them (easier to interrupt attacks and stagger opponents when they drop their guard).
For a medieval game with player based melee as opposed to character based melee (i.e. twitch as opposed to tactic), the melee combat system in Skyrim could have used alot more work considering it's one of the primary gameplay mechanics.