#1: Keep it simple.

#2: Output = Input. In order to toast two guys, you need to spend the time gathering the power for that spell.

#3: Force similar projectile rules to those of bows, guns, etc. Balances ranged spells very well.

#4: Newton's law: Equal and opposite. If a spell would blow someone twenty feet away, the caster also gets blown twenty feet back unless they use more magic to reduce or negate this effect. In which case, refer to #2.

#5: Have magic defense items! If they're very powerful, add magic penetration items. Arms races can be interesting to play.