Here's what I've gathered from my own trials

Priest Priest - takes a while to get started in PvE at proper speeds, unbelievable in PvP and ganking, can do doublepulls, maybe triplepulls in instances but it's too slow to grind elites with

Mage Mage - slow until Arcane Explosion at level 14, could use some twinking with +stam/+int/+spi gear, then the fastest sameclass combo of the bunch. Macro and bind properly to work better in PvP

Hunter Hunter - fast from start to finish, grinding elites from the moment the first one pops up, deadzone is a huge problem, multiple-target managment is hard but extra traps might help

Warlock Warlock - See priests, not that much experience with that combination, Warlock Priest (priest as main for healing and some shadow dealing, buffing) is a very good combo idea

Shaman Shaman - takes a while to get started, ranged only via elemental talents that take a while to get into, horrible itemization until Outlands

Druid Druid (balance specs) - takes a darn long time to get into, nonexistant itemization until Outlands, effective once 60+ and geared well, a good idea for PvP (hint: moonfire spam)

Shaman Warrior (war main) - shaman healing allows the warrior to make 5+ mob pulls right from the start, shaman buffs help a lot, windfury at level 32 combined with other buffs, totems and heals will make this duo a small raidboss