Warriors and paladins existed in real life. Wizards and mages didn't exist, the concept was created for fantasy books and games.
It would be unfair to mix real characters with fictional ones, as the real characters have real limitation, the fictional characters (wizards) have no limits. An unbalanced game is far worse than an unrealistic but innovative game.
Why shouldn't wizards be able to cast spells while wearing armor and such? To be a wizards means years and years of study. That means less and less time to practice other skills, like swordplay, etc. You'll see that, generally, genius aren't Schwarzneggers for a reason. The former has to spend hours studing, the later has to spend hours training in a gym. You can try to be both, but the day has only 24 hours, so if you go both ways, you'll never explore the full potential of following only one path.
So I say there's logic in existing a penalty in terms of strenght, stamina, wearing heavy weapons and armor.
If you used an armor (that weights very much, and I mean VERY), that would drain your strenght and stamina quicker, and you know that a tired person can't concentrate as well as when without that "obstacle".
And I don't agree there must be something innovative about magic in Oblivion. I mean, what can they think of, that wouldn't be futile? Drawing the runes onscreen? Arx Fatalis did that. So, when you ask innovation, try to give some suggestions as well.