In response to a comment posted elsewhere about the differences I saw between magic-users, clerics, and druids, I explain that each had a different relationship to Nature/Reality. Druids act as intermediaries between the mortal world and Nature. Clerics act as intermediaries between the mortal world and the supernatural realm of the gods. Magic-users, on the other hand, attempt to bend Nature to their wills -- to master and control it. The reason why MUs pay a price for their magical knowledge is because Nature doesn't "like" being warped to serve the whims of mortals and so does its best to destroy any mortal that attempts it. The problem is that, Nature, being impersonal and insensate in the mortal sense can't just snuff out meddling magic-users, really instead on the slow and scatter shot approach of destroying their bodies and minds, one piece at a time. After all, Nature is good at waiting.
Anyway, in my post about the druids, commenter richardthinks makes an absolutely brilliant connection: what if the constant re-memorization of spells is another minor way that Nature tries to foil the use of magic? I rather like this suggestion and think it works nicely with several thematic/stylistic elements I'm planning to import into my campaign setting. There are some kinks that still need to be worked out and this approach does require that, fundamentally, all magic functions more or less the same way, but D&D already assumes that anyway. The benefit of this approach is that it adds a flavorful gloss on the game mechanics that doesn't change the way the game works. That's a big deal in my book. Bravo.