Gladius - Is pretty much a must for Arena. Until you feel comfortable with enemy cooldowns, trinkets and dr's....etc.
Interrupt Bar - Also very good for what Olivia said above; Juking, and well interrupts.
Any other addon's you like....if you use a certain unit frame addon then bring it to arena, no point in reworking your entire ui just for arena's and it will make you more comfortable.

Cheers,
Larryfine

Macros Harvoc!
Macros!

All those spells you have bound to shift+ and ctrl+ can be built in to single macros. That way you can use two to three abilities on one button all modified by your chosen modifier. The cool thing about this is when you press a modifier your display will change to reflect the spell about to be cast.

@ Luke

That's what I was doing before I got the Razer Naga and it definitely wasn't working out for me because I couldn't remember where my spells are. I also couldn't see a spell's cooldown without hitting the correct modifier key to see the icon. Now that I only have 1 spell per keybind, things are much easier. The only problem I have is reaching the rightmost row of keybinds, which I will try to solve by switching over to the roaming index hand position variant.

Ah okay well that's why spell timers are useful. They're usually integrated into other Ui addons like DBM, Dominos and Milk Scrolling Battle Text. With a good UI set up you can have Icons that show cooldowns or have notifications set up with programs like milk.

http://www.curse.com/addons/wow/mik-scrolling-battle-text

I'd add Tidy Plates for a cleaner overview of people's nameplates.

Plate Buffs to show a massive icon over their nameplate when they are in CC or when they have popped a cooldown. smaller icons for other debuffs.

In the standard keybindings of WoW, unbind Tab from Targest Nearest Enemy and bind it to Target Nearest Enemy Player. That way you don't cycle through a hunter pet, 8 snakes from the trap and 4 totems while trying to CC someone.