a) Preparing. Which includes everything from studying what your opponent did (or might) say to collecting information and building arguments and evidence out of it.

b) Communication Skills (appropriate to the forum). Which may be public speaking face to face or writing skills online. On some of these debate sites it's 95% how clever of a wise@ss comedian you can be. Hey, that's a communication skill, too.

c) Courage. Say what you believe even if it's unpopular. Hear your opposition - because banning them is the coward way out. And if you lose or need to concede a point that too is real courage and in the long run you can grow from that - which makes you stronger.

d) Thoroughness. Whether fair or not if you don't respond to everything you're likely to have what you ignored back in your face.

e) Audience adaptation. Relevance depends on the forum.

f) Honesty (within reason). Because both your friends and enemies can smell honesty vs lies.

The ability to spell right, Gives links, Actually listen to what the person has to say and leave your emotions out is what makes a good debater.

The ability to accept another's argument as as valid as your own, and therefore respond in a respectable way.