For wannabe champs:
Necessary qualities, in order of importance:
Speed - Be able to very quickly assemble a team by knowing every set that comes your way and knowing how to paste sets. Have the mememon pastebin and the archive ready for anything you don't already know. Be capable of working with off-the-wall suggestions; if you're suggested, "Smeargle with only alliterative moves," you'd better be able to work fast. And for the love of God, start a new match instantly after a win or loss unless you have to evolve a Donnel or something.
Skill - If you can't reach 1600 in OU with your regular team, and some would place those requirements even higher, you really shouldn't champ. Learn common sets, keep up on the meta, memorize who outspeeds whom. If luck isn't on your side, no problem. But try to think ahead, embrace your shitmons, utilize everything. Champs have wrecked opponents with Toxic Orb Taillow, so take your Weedle like a man and git gud.
Fun - Sure, we want to see you win, but if you can help Snips reach his final form even though you've already got the win in the bag, do it, asshole. We're here to have fun. Do whatever you can to win, but make it as fun as possible. Got it?
Showdown knowledge - Do you know how to turn modchat off? Do you know how to show data about a given Pokemon? Do you know what's banned in OU and Ubers? Learn all of that for a streamlined champing experience.
Wit - Optional, but appreciated. Clever nicknames, knowing when to participate in chat and when to shut up, that's all you really need to know. Don't ask chat what you should do, don't try to excuse every bad move with a funny joke in chat, err on the side of keeping quiet.