I guess I will go over possible classes?
Priest: Amazing. Good surv. Clear's most magic effects that can slow a rogue down (ER,FB) Has a bubble to allow better movement for the healer. A clear out CC that is on a diff DR then the rogue. Has DoTs to keep a running target from restealthing O5SR etc (please dont DoT the main target otherwise, dont want to break that gouge =P). Overall a very good team comp. Cons: Bad priests go OOM quickly. Getting CC timers at the right team can be tough on a new team. Priest do go down fairly quickly. Hunters can give you quite the workout etc.
Mage: Pretty good with Rogue. Have the ability to keep someone CC'd for a quite a bit. Both benefit from food/water. The mage can keep one person away while the rogue deals with the other individually. You can reset the arena fairly easily. Mages have DoT's to interrupt bandages as well as Arcane Missiles. (Its fairly easy to get away and bandage) Cons: If the mage goes OOM, you are going to have a 2v1 situation quickly. Thus mages dont generally do amazingly well in long arenas.
Druids: I love druid/rogue teams. Druids are healers with a lot of movability, survivability, a CC and DoTs on different timers. They do pretty good damage, have a bear form for tanking a tough moment and can keep hunter pets off of you. Cons: Most of the heals are HoTs so shamans/priest teams can eat you alive if you dont cover well. Druids go OOM quickly against good teams so you have to really know your way around a spirit stick. Movement and positioning is huge with this team.
Shamans: Honestly shamans are just OP, so it makes this team okay. I dont think shamans have any super amazing buffs for rogues that allow this team to be really really good. Shamans just have amazing O and D capability, totems that can slow.. O dispels. Cons: There really arent many.. its up to how good your shaman/rogue are.
Hunter: Pretty good team in that hunter's are just good.. and hard to kill. Massive DPS, poisons from the hunter that are hard to clear.. a pet to keep the healer or whatever in combat. Overall a good comp that depends largely on the skill of the player.
Warrior: Not a great comp. You can burst one target down quickly by keeping them slowed and targeted..Teams with slows will make this team hard to win with. Warriors are just a rather gimped class at 19.
Warlocks: I like this team. Very offensive, have lots of DoTs, two very good CC's on different DRs. A good lock can make this team amazing. This largely depends on the skill of the lock. You can put a lot of pressure on anyone with this team.
Pally: Okay team, you get heals, HoJ (damn nice against mages though a good mage can reset and shut down the match). You get a stun for a key moment. I honestly dont like it that much. Pally's can do pretty good damage but you have to face the right team, and against some teams you really dont have that much of a chance.