Know your shamans.

Theres a huge degree of difference between the talent specializations available to a shaman. Depending on what tree they've chosen one shaman's playstyle can be totally different from another. So, suppose you come across a hostile shaman in the wild. How can you know what set of tactics to use?

Is the shaman wielding a two-handed weapon or two one-handed weapons?
If so, they're probably Enhancement. This shaman wants to get close to you in order pummel you with melee weapons. Stay out of reach, crowd control them when they get close and hit them from long range.

Look at the little ball orbiting the shaman. That's their elemental shield.
If it looks like a ball of electricity, the shaman is probably Elemental. They want to stand back and throw lightning bolts at you. Interrupt their spells and silence them as much as possible. If you're a melee class, get in close and keep hitting them to slow their casting.

If the shield looks like a brown ball of dirt and stone, the shaman is Restoration. They will probably try to use lightning bolts on you too, but with much less effect. Try to remove their shield because otherwise it will heal them as you attack. Otherwise treat them just like the elemental shaman, with special emphasis on interrupting their heals.

Rumor has it that all shamans will be able to wield two handed weapons after patch 2.2, but this guide should still hold true. Even though all specializations will be able to use the two-handers, only enhancement shamans will really have a reason to.