Hmm... fire... I like the idea, they have a sort of "tribal" look to them. The first one would be the best if it had straighter lines. Straight lines and angles can give an impression of power if used right. When you revise them, I'd color it red-orange and go for a very tribal look.
The second water one is best. It's like a drop reflecting light. Maybe you should make a new one in an attempt for it to appear like a reflection.
The metal ones look to random. It gives a sort of chaotic feel while metal is more of an orderly, easy to control thing. Try making a symbol with lots of pointy parts and perfect symetry(sp?).
I don't like the earth ones. Period. They'd make excellent wheels, but bad symbols. A symbol of earth should be sturdy looking, but nonsymetrical(sp?) and rough.
Finally, the air symbols also look to random. Only the third one actualy represents air. What about a nice universal symbol - like a tornado?

There, my uber-helpful suggestions. Though personally I believe the four/five/six element system is far too overused in magic. People try to think up opposing types of element, and it differs in every game so it's quite confusing. New World Computing devised an original system with Nature, Order, Chaos, Life, and Death to name one. Or why not make a magic system without spells, but instead different kinds of magic "powers" that can go towards healing, hurting, or other things. So much can be done with magic systems, yet they are so often overlooked. Hey, magic's not real, you can decide how it works.