I've often toyed with the idea of using a "real" magic system in D&D. I'd love to have a non-flashy, money-consuming, rituals-only Golden Dawn type of magic usable by mages, instead of the walking artillery currently in the game. Or maybe just an indian shaman or voodoo priest celebration style magic system.
I know various attempts were done for this in 2e (by Mayfair and TSR) and some OGL 3e products. But, in general, any of the people I've actually gamed with strongly prefer the flash-bang stuff. Which the game sort of requires. If a mage can't easily and quickly aid the party while in the dungeon, he just ends up being a burden instead of a helper.
My idea really would only work if magic was NPC-only, or the game was mostly low-combat, and probably city-based. A Golden Dawn practitioner would be pretty useless when faced with an angry ogre. But sometimes I wonder just how different the game might be if Gary Gygax had been heavily interested in the occult.