First off, hunters aren't a caster. They're more like a forgotten ranged class. If you think that affliction locks and frost mages are the only strong casters in arena at the moment, you're pretty delusional. Fire MLS is still a viable comp, though most people go with frost mls because its much easier to pull off. Arcane/Destro/Demo/Boomkin are all bad specs because they require too much casting. None of them have reliable instants outside of Boomkins, which puts them at a disadvantage against other casters. Ele shamans are very strong in 3s/5s/RBGs, as they can easily line up good burst and have a reliable knockback. Shadow Priests have the strongest comp at the moment, Shadowplay.
The biggest reason destro/demo locks aren't strong is that they have no dispel protection. Affliction locks can fear anyone and be sure its most likely not going to be dispelled off. The other big difference is that affliction has instant fear, the other two specs don't have it. Destro's damage is casted mostly and pretty bad, and demo relies too much on spells with long cast times.
EDIT: Also, the only teams that I see being competitive at the tournament are the american/eu teams. On the american/eu teams, there is only one melee (dk), and warlock/shaman on each team.