Knowing other classes and what they do in many situations.
Keybinds are necessary for any player planning to go 1800+
Keybinds and a good player make the world of a difference when it comes to playing arena and succeeding. Since my partner quit when WOTLK came out, I haven't had any urge to arena since my server is one of those backwater fail ones with a pool of talent, even though warlock/druid isn't anywhere nearly as good now.
As everyone says here, find a partner you can mesh with well, get a comfortable keybind setup, and put in a lot of practice, it'll click eventually.