The math teachers that I like the most are the ones that make the subject they teach look very easy. For instance, my abstract algebra teacher makes all the definitions justified and look very simple. He always finds the simplest arguments for every proof, and since he does everything in the right order it just seems to flow, and we almost could guess every proof that he's about to give us, because they all seem very natural.
I'm not sure I know how to summarize this quality exactly, but I'd say that every math teacher should be able to make the subject he's teaching look simple and easy by finding the simplest proofs and doing all the necessary intermediate steps that lead to a more difficult theorem.
