WoW Insider talk about what they consider to be essential addons when you are battling in the Arena.

There are literally thousands of addons out there, and some of us quickly develop an almost unhealthy reliance on all the bells and whistles. I'm a firm follower of the minimalistic movement: if I hardly interact with a particular addon during a typical game session, it disappears from my AddOns folder, no matter how nice or "useful" it seems. I prefer quicker loading and response times, to conveniences that are situational at best, thank you.