I personally prefer the Summoning/Outsider Pact method of spellcasting to the natural magic/inherited power method. Anyone can become a spellcaster as long as they are willing/able to either bind lesser spirits as their slaves/allies or they offer themselves to greater spirits in exchange for power.

Alchemy comes in as a close second since it seems to have a long involved process to it -- rather than just waving your hands and saying a few words the caster actually has to brew up elaborate potions and follow strange formula for their result.