Offense versus defense
The first thing to remember with team composition is that offense is always more important than defense. Timing large bursts of damage together or being able to crowd control the opposing team into oblivion is much more important than trying to make a team around, "Well, if we get into trouble, we'll have Ice Block, Barkskin and Hand of Protection."
If you've played arena at all since season six, you'll notice that spell cleave compositions (two DPS spellcasters + a healer) is an incredibly popular team. Why? It's successful.
Spell cleave (wizard cleave)
Elemental shamans are fantastic at not only creating a large amount of spell damage, but also providing a way to stop heals (Wind Shear). They also provide a rather large bonus to teammates, with Totem of Wrath. Elemental shaman generally do much better with spellcasting teammates than a sword-slinger.
Of course, we're talking about a burst team whenever we talk about an elemental shaman. You don't usually try to wear down your opponents when you have someone capable of doing 10,000 damage in mere moments in your starting gate. Many other types of spell cleaves (or wizard cleaves) exist, but most of them involve some combination of warlocks, mages and elemental shamans. I see an occasional boomkin here and there too, and that is awesome.
It's important to realize what your team is trying to do versus what the opposing team is looking to accomplish. If you are a nuke-them-up team, you need to get off a few successful crowd control spells or silences while you mash buttons furiously for a death.
A double-healer team is about as far away from a burst team as you can get. Usually, the game plan is to outlast the opposing healer's mana. Games might be two to three times as long as a "normal" arena game when teams run a two-healer setup.
Choosing two healers that work together well is of utmost importance. While I've seen some restoration shaman + restoration druid + random DPS class work, it's pretty rare. Starting with a defensive healing pair (discipline priest + holy paladin) or offensive healing pair (discipline priest + restoration druid) is what I normally see.
And that brings up a good point -- discipline priests are a premium in this type of composition. Mana Burn is just too valuable a tool to pass up. The added DPS that discipline priests can pound out when an opponent is low is just icing on the cake. And the offensive/defensive dispels are candles. Happy birthday to you, dear Dispelsalot, happy birthday to you.
Having a DPS class that complements your two healers is really where you take the team to the next level. While a fire mage might sound like a good idea, usually a DPS class with a reliable Mortal Strike is best. It also helps if the class benefits from staying on the offensive -- like an arms warrior.
While the sentiment against "cleave" teams has shifted a bit since season three, a team supporting two physical DPS + one healer is normally thought to be among the easiest of team compositions to pilot. And for good reason -- tunnel vision is practically defined by this archetype.
But that's not a bad thing. Really, it's not. Tunneling one target is fun. If you don't believe me, go roll a physical class that has Scorpion's "Get Over Here" move or another unnamed class with teleport stuns. Team up and go to town on some unsuspecting clothie. The moment you see his health drop from 100% to 20% to 0% in the matter of mere moments will have you hooked on arena forever.
The healer on a cleave team is usually in a pretty awesome position, too. Early on, physical DPS teams tend to not be threatened or take a lot of damage off the bat. A holy paladin can run in and Hammer of Justice a target in place. Restoration druids can stealth into a great position without a ton of problems. Shamans can have ample time to perfectly place totems.
The "balanced" archetype is one with physical DPS + spellcasting DPS + healer. This archetype usually doesn't have severely poor matchups. While the above teams are often fighting rock-paper-scissors style, a balanced team composition will sometimes just mushroom everything fairly well. In The Burning Crusade and for much of Wrath of the Lich King, rogue-mage-priest dominated the arena charts on every battlegroup in nearly every season.
Of course, because balanced team compositions don't have absolutely terrible matchups, they forfeit overpowering anyone. Watching a balanced team play arena might be more exciting than most other games -- they usually are very close, no matter if it's a win or a loss.
The downside to playing a balanced team composition, however, is the high skill factor involved in the team. Knowing how to choose a physical DPS class that complements a spell DPS class well, and then how to mesh that with a healer, is a true test of metagame understanding.
Want to ascend the arena ladders faster than a fireman playing
Donkey Kong? Check out WoW.com's articles on arena, successful arena PvPers, PvP and our arena column, Blood Sport.