I think that each element should have its own pros and cons.

My ideas, some taken directly from Diablo II:
Fire is powerful and straight forward.
Lightning seeks out targets and can chain between them, but is more eratic in its damage.
Poison is very powerful, but acts slowly.
Freeze is weak, but immobilises the target.
Acid is average, but disolves armour.
Spiritual Damage is strong, but only versus intelligence (ie. doesn't do much to animals/undead).
Holy Damage is strong, but only versus evil.
Corruption is strong, but only corrupts the good, making them deranged and likely to attack their allies.

I like the idea of the Lightning spell. Lighning's pretty! <img src="/ubbthreads/images/graemlins/smile.gif" alt="" /> The only issue I have with it is how many bosses will be outdoors? That would be the main reason I'd purchase such a powerful spell - to take out bosses. But why if I can't use it on them? Is there a variation that might work?