In order of importance...
He needs to know the nusach. In this, I am a strict traditionalist. Avos, Aleinu, Kol Nidrei, Kaddish, among the other set pieces, must be done the old way.
Have a good voice Do you want to spend an entire day listening to someone who can't sing?
Know singable, non-boring tunes The piyutim are better when everyone sings together. A chazzan who can't inspire this sort of participation shouldn't be allowed to lead the service.
and be able to read the words correctly (in all their particulars and minutia) Obviously, the Chazzan needs to know how to read, but many congregations will tolerate a chazzan who changes the meaning of a verse by accenting the wrong part of a word, or by pausing at the wrong place. That's not acceptable.
