Just thought I'd put down some ideas for tactics in small groups and low level instance running. This isn't the complete guide, just various tips and tricks.
The same roles are mostly valid in higher level instances but class specialisation has a noticeable effect; no more rogue tanks in higher instances please!
In any group there are certain roles to be filled. These can be filled by various classes at times, but obviously some classes suit a role better than others.
A tank is the character doing the up close and personal melee combat with the monsters (MOB, NPC). Their role is mostly to soak up damage and prevent the mob from attacking other group members. Other aspects of this role are preventing spell casting and preventing fleeing. Damage dealing is NOT the primary requirement here, but is important in "holding aggro".
Warriors and Paladins are the tank of choice due to much better armour than other classes. Bearform Druids get boosted armour and abilities to handle this role well. Rogues, Hunters and Shamans can tank lower level instances if there isn't someone else available. No spell casting cloth wearer should ever be the primary tank. Warlock and Hunter pets can tank to some extent but have real aggro management problems.
Similar to the TANK above, the OFF-TANK is a secondary Tank. They have a more specialised role in groups. The off-tank's job is to manage any ADDs (additional monsters) or mobs that break away from the main tank. Spell caster interruption and flee prevention are high priorities here. A general awareness of the combat zone is needed. The Off-Tank must be alert for ADDs or re spawns, capturing their attention and either managing it or drawing them to the main tank.
Almost any non- cloth wearer can be a suitable off-tank in lower level instances. In larger groups it is a warrior, druid or pally that takes this on; in smaller groups Rogues or shammys work well. Hunter pets are often acceptable off-tanks and some of the hunter and mage abilities are useful for mob management.
This is a role where a solo player moves away from the group and attacks the next mob to bring them back to the main group. It generally requires some form of ranged attack, so almost any class can do it.
Pulling is an art form; do it wrong and you will draw far too much aggro and wipe out your group. Done well and just the selected mobs are drawn away from their friends and onto the party's main tank.
The outright best class for pulling is the Hunter. Hunters have a number of abilities that make them great pullers, the main one being the Feign Death ability. At least one of their attacks is a slowing/stunning missile shot and they can place traps and their pet in the way of any incoming mob. They can also then feign death to pass the pulled mob over to the main tank. A failed pull done properly will kill only the hunter and not wipe the whole party.
The next best pullers are probably whoever is the main tank. There are a couple of special varieties of the pull though:
The sheep pull :- a mage uses the pollymorph spell to "sheep" one of a group of monsters. The rest of the mobs then charge straight for the mage. It is the job of the tank to get aggro off the mage really quickly.
The SAP pull:- a stealthed rogue uses SAP or another stunning attack to take out one of a group of enemies.
Damage Per Second. People with this role have one main function: making mobs dead really quick. The trick to this is that you don't want to just spam your most devastating attacks, since you will draw more aggro from the mobs and possibly take them off the tank and onto yourself. This is bad... healers are mostly too busy looking after the designated tank and healing everyone just isn't efficient. DPS classes that pull off the main tank too often wont be invited back!
The stand-out classes here are Rogue and Mage. Both have awesome damage dealing abilities... but as noted that can also be a problem. If you moderate your damage output so that you are running at about 80% of max you should be ok. Other good classes are cat-form Druid, shadow spec priests, Shammy, Hunter, some warlock pets and warlocks.
Damage over Time (spells). Mostly this is the domain of the priest and the warlock, but other classes have some skills in this area. Damage over time effects are useful for weakening enemies and killing off fleeing mobs.
Can generally be used on multiple mobs at once without pulling them off the tank or off-tank. Be aware that most of these effects will break crowd control.
Crowd control is an ability that only some classes are good at. It involves assisting the off-tanks in removing any ADDS or just handling the extra mobs above and beyond what the main tank can handle. Rogue sap and gouge abilities, Druid root spell, mage freeze and sheep as well as various hamstring attacks are all aspects of this role. Hunters have traps that can immobilise or slow mobs as well. They get much better at higher levels but are initially quite limited. Priest and warlock FEAR abilities are also useful but run the risk of causing more problems!
Most spell caster classes have some role to play here. Apply bonus effects on other party members outside of combat. Remember to boost pets if applicable. Some pets buff as well.. warlock imp most noticeably.
Not the most glamorous of roles perhaps, but essential none the less. Priests, druids, shammys and paladins are just about the only real contenders here, though anyone with first-aid skill can assist. Without your healers, a group will die often and quickly. Another aspect of healing is of course resurrection. Warlocks have an interesting role to play here, in that they can create stones for healing and soul-storing, as well as having the abilities to drain life to keep themselves alive.
When healing, look after the main tank as much as possible and only assist others if the tank is managing well. Also note that your highest level spells are not always the most efficient.