Divine spellcaster's have to roll a Will save to cast spells (DC= 10 + twice the spell level). Clerics don't have to make the roll when they use spontaneous healing or Domain spells. This helps offset the fact that these spellcasters also have decent combat training, as well as the ability to wear armor.
Divine spellcastes CAN attempt to cast spells beyond their normal level (higher level spells, extra spells beyond the usual amount per day), but this doubles the DC to cast. This DC increases by +2 for every attempt to cast beyond their means for the day, successful or not. If they fail, they take damage equal to twice the amount they missed the roll by.
Divine casters also don't have to memorize spells.
I have no problems with other spell casters. They're only as good as their creativity.
EDIT: I don't give wizards the free two spells per level. They can research any spell at their disposal (most have access to a lab and library, though I do control what level of spells are available at what labs), it just isn't assumed they succeed. Tends to work well, given the amount of scrolls and spellbooks that abound. It also works in the players favor at times: The spells they gain are proportionate to their down time, not their level progression. If there is a year or so between adventures, the wizard may actually learn more spells.
Given how often I've heard of wizards (both PC and NPC) who's spellbook contains every spell in existence, it's really a minor hitch.
It also allows me to throw in 'corporate espionage' type adventures for a bit more fun.
