Well, not only did Morrowind have non-regenerating magicka, but it also had it so that spells had a chance to fail, and at lower levels they almost always failed. These things combined made trying to be use spells far too much of a hassle for me to even bother with. But constantly regenerating magicka in Oblivion made things a bit too easy for my tastes. I think I would like it if Skyrim had non-regenerating magicka, but didn't have the dice-roll spell system Morrowind had. I want the challenge of only having a limited amount of magicka, without the hassle of wasting it on failed spell after failed spell after failed spell.
Personally, I enjoyed that system. I would spend a lot of time ensuring that my spells had a good ratio of power to chance-to-fail so that I wouldn't get into a loop like that.
Really, I want much of the magic system from Morrowind back. I really enjoyed being a mage in Morrowind because a lot more thought went into the process. Of course we keep the things like not having to ready your hands (that was silly), but I really would like to have back spell failure chance, personal enchanting, and the ability to enchant stuff like paper to make scrolls, although I would like it if that system was expanded. The best scroll I made in Morrowind only did something like 20 fire damage on touch.
As for mana, I suppose we are at an impasse. Either we make mana regeneration slower and have no spell failure rate, with the challenge being that one must think of which spells they are going to cast before they do so in order to maximize effectiveness, or we have the same fast regeneration and have a spell failure chance to offset this.
The reason I believe we should return to spell failure rates is that the only other way to balance spells is to have ridiculous caps on what spells you can use at what level of the skill. Its totally insane that I can't use such-and-such spell at rank 74, but can at rank 75. How more arbitrary can one get?