There's one primary difference in the LFG pick up groups that work well and those that do not: the use of crowd control! Blizzard has forced the content to be even harder than it was in Wrath so that all those abilities become useful again, including crowd control. Even in the normal dungeons, the amount of damage that rolls through a tank with two mobs active instead of four is a huge difference. This can prevent you from drinking every pull, which slows the group down and may cause impatience (rightly or wrongly). I recommended taking a PUG group by the reins when you queue up and find a tank or group not starting with the right crowd control. But most healers do not know how to do this without having been a tank first, and usually in pre-Wrath days. Information is power, and you need to know what to expect if you are calling the CC shots from the back.
The first thing to realize is that every class does indeed have some crowd control. However, some are far more useful than others because they last longer or apply to more types of enemy mobs. Hence, the crowd control abilities of hunters, mages, and rogues are highly valued and are a nice extra task for the DPS classes. Let's begin with the DPS classes, which generally have the best crowd control.
Mage - Polymorph - This ability turns a beast, critter, or humanoid into a small animal (sheep/turtle/etc.) for 50 seconds at maximum level. Generally used with the moon mob marker in most groups, although this assumption is subject to change.
Hunter - Freezing Trap - A freezing trap can trap any mob for 20 seconds, but requires the mob to run into the trap. Thus, the hunter must generally be experienced at positioning this trap in front of a mob that will run towards a pulling tank. The freezing trap can hit any type of mob, and a Survival Hunter has two talents (Trap Mastery and Resourcefulness) that extend the duration and reduce the cooldown of the trap to enable chain trapping indefinitely. Make sure to know whether a hunter can chain trap before assuming they can.
Rogue - Sap - At maximum level, sap can affect humanoids, beasts, demons, and dragonkin. Note that sap must be done prior to combat, so the rogue must lead with this ability. Sap lasts 60 seconds at maximum level.
Warlock - Fear, Banish, and Seduction - Although warlocks have three crowd control abilities, each of them are risky in instances or severely limited. Fear is repeatable to make a mob flee in fear infinitely, but in an instance this will generally lead to accidental pulling of more groups of mobs and a wipe. Hence, fear is very limited in application. However, the Glyph of Fear allows a Warlock to make a mob cower in place rather than run around wildly, which makes this one of the best CC abilities. Banish only applies to demons and elementals, but lasts for 30 seconds. Seduction is an ability only usable with the succubus pet, and it shuts down humanoids for 15 seconds. Enslave Demon is also another option against demons, which makes the mob a pet for up to 5 minutes. Thus, Warlocks bring a nice mix of different abilities to the table.
Turning to healer classes, each class has one primary ability but most are limited in duration or the classes of mobs it can affect. However, these abilities should not be forgotten when the four pure DPS classes are not available or need supplemental crowd control.
Paladin - Repentance - This ability is only available to Retribution paladins, so healadins will not have this ability (but you knew that already if you are reading this blog). However, Repentance lasts a minute and may be repeated indefinitely, and is applicable to humanoids, undead, dragonkin, giants, and demons. A great ability to have at hand if the Paladin is of the correct talent spec.
Shaman - Hex - Shamans of all varieties can turn mobs into frogs, much like the polymorph ability of mages. However, only humanoids and beasts may be hexed. The ability is repeatable, so the mob can be taken out of commission as long as possible.
Druid - Hibernate - This ability may only be used on beasts and dragonkin, and lasts for 40 seconds. While hibernate may break early on mobs of equal or higher level, so the druid will need to stay diligent. This means bear tanks typically cannot count on this ability as a useful CC, so this may be better from moonkin and healing druids.
Priest - Shackle Undead - As the name suggests, this is a repeatable ability that works only on undead mobs. This was great in Icecrown Citadel, and not so much thus far in Cataclysm. Priests also have Psychic Scream, an area of effect fear, but this is even more dangerous than a warlock fear.
Now we have the two remaining melee DPS and tank classes, neither of which have great crowd control abilities. But just in case it becomes relevant...
Death Knight - Chains of Ice - For 8 seconds, a mob's movement is impaired by 60%. The short duration of this ability makes it not all that useful unless a pack of mobs is being kited away. Death Grip could also be used to pull one mob off the tank, but incoming damage to a non-tank melee DPS death knight will usually be a more significant drain on healer mana than just leaving the mob on the tank.
Warrior - Stuns including Charge, Intercept, Piercing Howl, and Hamstring - None of these stuns lasts more than a few seconds, so these abilities are only moderately useful as damage-slowing crowd control. Better than nothing, but not by much.
So now that you know the types of crowd control, the only other thing you need to know is how to mark mobs. This is a very simple process now. Target the mob, right click on the portrait of the mob, and select the mark from the drop-down menu. Once you determine what 1-2 CC abilities will be used on each pull, the only other important point is to ensure that each player responsible for a CC spell knows which mark applies to their mob. For example, a star could mean rogue sap or hunter trap, but just clarify it at the beginning and then it is clear what you mean with the marks. Also be sure to use skull and X on the first and second kill targets, which should be the highest threats to your group survival after the mobs removed by crowd control. Studying the mobs of an instance will help you be a better leader and never be afraid to ask for advice from tanks and DPS who may know kill orders better from doing it before.
Tanks are not the only class that can lead and can mark, directing the group to success through crowd control. Before a train wreck LFG pick up group puts you back in the queue, take the reins and lead the group. The tank will probably appreciate it and things will go smoother for healing as long as the CC assignees do their jobs. Until next time, use the abilities Blizzard gave you!