Thing is, many if not most runes substantially change the functionality of the skill. Ray of frost, for instance, can be a straight damage dealer, a 0-cost left-click spell to be used while you wait for your arcane power to come back, a close-range circle of cold damage that lasts as long as you hold down the button, and this other thing that "leaves behind a patch of ice" that deals 193% damage to anything standing there over a period of three seconds. There may be better examples; I just picked the first thing I thought of. I would use slow time and that barbarian one where you grab an enemy with a chain and pull them toward you, but I should really be somewhere else right now.
Angora wrote:I'm starting to lean towards the Witch Doctor.
Doctor Whooves

He's got some really cool stuff.