Balance, and something David Eddings calls "the Will and the Word." In his book about writing the Belgariad, he says magic should be limited by the mental fortitude of the wielder or else every mage will be a god, and the constant use off magic would realistically be very mentally straining. In other words, you can say the incantations as much as you want, but unless you have enough mental fortitude or focus (the mana bar) you can't cast.