Personally, I think that magicka should regenerate slower, but I also think that you should be able to cast spells when you're low- so long as doing so taps directly into your HEALTH or ENDURANCE when your magicka is too low. Like debit card, sort of. Just like in fantasy literature when an exhausted wizard digs too deeply to cast a spell it has adverse physical affects on him.
This idea has good merit I think also with the new runes system a magic user will be able to prepare for a battle so that he/she has stuff up and ready before even going into battle. Tapping into your health and endurance would make it more visceral and force you to choose more. Also this could allow a caster to cast a mother of all spells and leave them with no magika and little.
I still like the exponential growth with respect to willpower such that magika regens very slowly, slower than Oblivion, and at high levels of willpower regenerate faster than Oblivion. If you have 80 strength and endurance you rarely had a down moment as a warrior and you missed few times. You cast a couple of powerful spells and you had to run around like a chicken with his head cut off waiting for your mana to regen.