Which Duo to Choose?

[Renault]

Shaman's strength is slows, buffs, debuffs. If you don't use all three of those, then you're not getting the most out of your duo.

Shaman's main weakness: only CC is roots.

If you are thinking endgame viable duo partners for raiding, then you have a lot of options:
(1) Monk - been discussed. There are a zillion monks on this server.Make leveling easy with good dps and, most important, ability to pull.
(2) Any other melee - SKs can also pull, Paladin can try to lull pull, Bard can lull and mez, Rogues can sneak pull, Halfling warriors can sneak pull.

You can also just choose areas to xp where you don't need to split many. Up until the 40s, shaman can just slow tank, so a rogue could backstab and take them down. Bard is a non-traditional choice but actually rounds out the shaman pretty well, are raid desired, and can tank fine through the 30s and probably 40s.

(3) Any caster but wizard, druid and cleric (unless you want to stick to undead)
Enchanter is the obvious one, with mana regen, charm, mez and lull. The winning duos for endgame are usually considered enchanter/cleric or monk/shaman, so you are just doing things a little differently.

Necro for reasons partly discussed - pulling, charmed pet, mana twitch, high damage dots, etc.

Mage - OP pet, can haste and stat buff, slow target, root adds, dot, etc.