If you consider the manner in which a shaman can do damage, you may notice that the majority of their spell damage can be done at instant speed (shocks/shield), an instant cast totem that sits there and throws fireballs, coupled with DW, high AP gear, Strength/Agility totems, fast magically enhanced weapons, and ways to replenish mana while fighting, the results I am seeing are not surprising at all.

Rogues cant compete against that, because they lack the spell damage(even though they benefit from totems).
Mages have more spell damage, but not so much more that it makes up the difference of DW shaman melee.
Warlocks dont get the full benefit of their DoTs because things die too quickly.
Hunters (most) go oom too fast to compete there.
Warriors got rage generation nerfed, and havent been uberleet DPS since.
Shadow Priests can almost compete IF the mob lives 12 sec or more, they have 400+ damage (or more) on their gear, and they manage not to pull aggro and die. (personal experience here, and yes I did die frequently because MB crits ALOT with my current spec :) )