There's problems with Mounted Combat:
Archery is balanced by movement speed. Mounted Combat would trivialize it
Dual Hands system also makes it difficult. Lets say you had a Sword in Right, Shield in Left. If an opponent was attacking your left, your sword arm would be on the opposite side, limiting attacks.
What about casts? Would self casts affect the horse or player, or both?
Would it be easy to accidently hit the horse with a touch/target/aoe spell?
Actually, Archery isn't balanced by movement speed in a way Mounted Combat would trivialize. Archery's balanced by:
Very small angle of tolerance to hit a target. A melee warrior has ~45
o, while an archer's can be reduced to a handful of
minutes of a degree, depending on distance.
It's difficult enough to hit a fast-moving target when you are standing still. It's even harder when you are on a fast-moving, constantly shaking platform (Such as a galloping horse). So, you still take a massive accuracy hit when fighting from a mounted platform.
And for the dual-hands thing... So what? Position is important in combat. Either attack with what options you have (It helps to look at the target. Can't hit someone to my left if I'm looking right, mounted or not), or steer around to get them on your left. I hate to parrot it, but if Mount&Blade can handle it, Skyrim should be able to do it
even better. It's also not that hard to look over...
As far as spells: Self-casts should affect Self, not Horse. It should be possible to cast spells on the horse with Touch spells. And the horse should be as vulnerable to target/aoe spells as you are. Just control where the camera's facing (Just like nonmounted combat) and it should all be obvious, and not an issue.
And anyone trying to fight on a horse using anything more than their knees to guide it needs to go back to Horsies 101 class.
Mounted combat is balanced by the price of the horse, area you have to maneuver, and the fact that even on a horse, there's a chance your foe is twice your height.