One of my biggest complaints with Morrowind is that I had to pause the game to dig through my magic to find the spell I wanted to use- often in the heat of battle. This completely killed immersion for me. A "real" mage in the middle of a battle has to think quickly on his/her feet. There is no "pause" button in warfare, and using magic should not change that. Now, Fable had a very useful system in which spells could be mapped to a hot key interface that allowed for very quick use of spells. You simply memorized the direction on the d-pad that each of your most used spells was mapped to, and when you wanted to change, merely pushed the direction and a button simultaneously.
Now, obviously Fable was quite a flawed game for those who like RPGs, since it really did not have an open world. But combat-wise, I think it did well with some aspects of magic. Particularly the ability to quickly choose spells without having an immersion-killing pause in the middle of battle. Anyone else feel that something like that should be implemented in Skyrim?