For the non-shamans
It seems that often Shamans are thought of as nothing but Priests with shocks, which isn't true. We straddle the border between 3 worlds: we're good (but not great) damage-dealers, we're good (but not great) damage-takers, and we're good (but not great) healers, with those abilities becoming great depending on which talents we choose. A shaman who specializes in Enhancement becomes a very good melee damage dealer. So don't be surprised to find us up with the warriors, at least at the start of the battle.
Elemental and/or Enhancement shamans typically come in guns blazing at the beginning of a fight (dropping totems as needed), then gradually fall back to a healing position to keep everyone else alive. Warriors & Rogues will always be at the front kicking ***, Mages and Priests will always be in the back throwing out spells and heals, but a shaman will be where he's needed most. Don't expect us to hang around in the back waiting for you to bleed. Not only is that boring for shamans, but it's also a waste of a slot in your group. If you want a priest, get a priest.
Shamans are (as at patch 2.1) the best healers for large groups of people taking damage in raids. This is thanks to that mightiest of all spells, I am talking, of course, about Chain Heal. Chain Heal is not only extremely mana efficient(512 if you're specced for it), but also extraordinarily good at utilizing +heal. Chain Heal has for a long time been the shamans trademark spell, but has only recently become the best healing spell in the game(imo ofc^^). Well, i mentioned its efficiency with +heal, but what does that actually mean? Well first of all it means that it scales very good with gear. Compared to The other Shaman heals Lesser healing wave: 43% of your+ healing us being used. That means that a LHW that would heal 1500 now heals up to 2145, if u have 1500+ healing. Our big heal is slightly better, using 86% of +healing, But Chain Heal... Uses a whooping 126% of your + healing. Yeah its true! To stop the number-crunching and finish this of, when you have good gear chain heal is absolutely awesome. Use it and you will find yourself on the top of the healing meters in raids every time.
Shamans can make capable off-tanks. If you ask a shaman to do this, however, their capacity to heal is seriously reduced, as much of the time they will be forced to heal themselves. Shamans can main tank instances, but often at such a high level that the shaman is unlikely to see any point for being in the instance. Blackrock Depths is generally considered the highest level instance a shaman can hope to tank.
Enhancement shamans will greatly increase your group's melee DPS, because of Unleashed Rage.
Much like Druids, shamans should bring multiple gear sets, and use mods such as ItemRack to quickly toggle between them. A versatile shaman should not hesitate to go between 2H weapons, 1H and shield, or stepping back into a healing role, with gear choices that complement these tasks.
For the shamans
Rogues USED to have the complaint about Windfury totems replacing their poisons, However, as of this date Blizzard has fixed that problem and it no longer occurs. However, it's always best to ask your rogues if they prefer Windfury Totem or Grace of Air Totem.
Warriors want to have all aggro. Getting hit is good for their rage and they have the best armour anyway. Frost Shock generates additional threat and can steal aggro from them. Avoid using it for damage dealing.
If you're not off-tanking (or tanking), consider not using Lightning Shield. Since you're not tanking, you don't want to maintain aggro when you get it, and Lightning Shield will only do damage in this case anyway, increasing the difficulty for the tank to re-acquire aggro from your attackers.
Mages can Polymorph, but be wary of your Chain Lightning spell as it can interrupt the often-wandering sheep. We can't DPS or CC properly if our sheep gets interrupted!
Squishies (Mages, Priests, and to a lesser extent Warlocks) might like a Frost Shock to get their mob's attention if they get aggro. This is especially true when they have Crowd Controlled a mob as it will run right at them when the CC wears off. Frost shock is best for this, because it is instant, doesn't have a DoT, and generates extra threat to boot.
When you're in an instancing group, watch the puller's target and get ready to Earth Shock. There are times where the puller shoots his bow on a caster, who starts casting and ruining the pull. Mages' Counterspell can help, but your Earth Shock has a lower cooldown (30 sec. vs. 6 sec.). Consider this when you have a mage in your instancing party.
When in doubt, Grounding Totem in boss encounters.
PVP with a shaman
* Spec vs Spec guides below (You vs Him)
* Enhance vs Resto - Same thing as a priest or a paladin. Conserve your mana and wait for him (he will use it) to use Nature's Swiftness. If you're quick you can purge his Nature's Swiftness right off him. Go straight for dps close range and you will have him. Remember, go two hander since he will most likely be using his spells rather than melee. Earth shock any of his heals. Searing totem is preferable for slowing down his heals.
* Enhance vs Elemental - Same thing as a mage. Earth shock every move he does. Purge any of his buffs and stay very close range. Earthbind Totem soon as you're next to him.
* Enhance vs Enhance - Interesting Fight. Depending on Gear, keep him kited while using most of your mana. Once it's close to gone just go straight to melee. If you pile on a Stormstrike with a few Earth shocks as soon as you see a Windfury pop he won't have time to do a quick heal.
* Elemental vs Enhance - Use Earthbind totems and shocks. You have the bigger mana pool so you can afford to use shocks. Elemental Mastery devastates if you can stop him getting close range for you to cast. Pile on the DPS and make it a quick fight. Be sure to keep grounding totem up when you go for heals. Earthbind and frost shock kiting helps a lot.
* Elemental vs Resto - Earth shock fight. This will most likely turn into who can Earth shock the other's lightning bolts/heals. Purge his Nature's Swiftness and use your Lightning Overload fast. Be sure to cast grounding totem before heals and keep searing totem up to slow down the enemy's casting.
* Elemental vs Elemental - Should be quick and simple. Earth shock most of his attacks and again Purge his top talent ability. Grounding totem before heals and searing totem to slwo down their casting.
* Resto vs Resto - Purge his Nature's Swiftness and Earth shock his heals. Brace yourself, this one will be a long fight. Be sure to take down any totems and keep your totems up. Get your timing with mana tide down right (which you should have as resto). Use water shield because its mana efficient and most shaman will either purge it (wasting mana), hit it and purge it (giving you mana and taking their mana) or leave it (giving you mana for this enduranc fight)
* Resto vs Enhance - I must tell you now. If you're attacking a enhance shammy as a resto, don't count much on your survival rate. Do not! let him in melee range. Purge any buffs. Kite with Frost Shock and Earthbind. Nature's Swiftness + Heal when close to death. If you mimic paladin tactics then you can kill him, but remember that you don't have a bubble so running away won't be easy. Pray that you're lucky and they dont get insane windfury crits on you.
* Resto vs Elemental - Earth shock fight. Earth shock every bolt he casts. Purge his Lightning mastery and use Nature's Swiftness fast. Grounding totem for heals or when your shocks are on cooldown. As a resto shaman, you should have more mana and mp5 giving you an edge in this fight.
* Water shield is your friend.