This is just to share my personal experience on the fun and fustration of running instances with pick-up groups. Maybe it is sterotype, but I have noticed that certain classes display certain traits. Feel free to chip in!
Druid
They are not very common in the game, but feral bears, cats and tree-of-ife druids certainly know how to do their jobs of DPS, tank or healing very effectively. So far, I have only seen ONE joker who blast moonfire in sunken temple until he goes out of mana, switch to bear and charge beside the warrior tank.
Hunter
Despite my other thread about tanking with hunter pets, I must say good hunters are generally underappreciated by the 5-man team. 2 of my best in-game friends are hunters and they, together with good hunters I have grouped with, were willing to send their pets against an elite mob while we were engaged with the other 2 or 3 or when the healer is attacked.
Mage
My main is a frost mage, and I must say some mages don't know how to protect themselves and let the tank gain aggro. AOE spells like blizzard, flamestrike, arcane explosion are to be used only when necessary, not so that you can top the damage meter. I always reserve my frost nova to save only the healer and myself and if the re-sheep the mob after the 50 second timer. Polymorph is more powerful than casting an additional pyroblast, so use it even though it lowers your chance to top the damage meter. As a reminder to other players, please DON'T attack that sheep!
Paladin
Most of them I have grouped with are very good tanks, or retribution specced who does good off-healing. A protection specced paladin is the best group-tank and mages can safely AOE down mobs without crowd control. On the other hand, less than half of the paladins likes to "try something new" by pulling and tanking a group too large to be taken down by the party or for the healer to heal fast enough. For goodness sake, limit your tanking to not more than 6 mobs, getting killed is a fustrating part of the game, not "something new".
Priest
Sadly, everyone expects them to be good healers and nothing else. Almost all of them I have grouped with play their roles in shadow form or healer very effectively, but seldom appreciated by anyone. I strongly urge all players to keep an eye on the priest's mana supply and if they ever get hit, do something to save them.
Rogue
Half of those I grouped with are good with stealth tactics, sap and dishing out damages when needed. The other half are the ones who break my sheeps more than any other player I have grouped with. The extreme example is 2 rogues in my 5-man group in Hellfire Ramparts declaring they don't know how to use distract and lose aggro by vanishing.
Shaman
Definitely the best 5th player when you cannot decide/find who to fill in the last empty slot. So far, the best role I have grouped with seem to be that of a healer when we don't have a holy priest or resto druid, although multi-role fighter is more common.
Warlock
I have not grouped with them often, so not much to comment on. Their DOT enable them to top the damage meter sometimes, and voidwalker pet is a good off-tank too.
Warrior
Usually assigned the tank role, another unappreciated role in the game. A warrior is the best single-target tank (and hence boss tank). Tanking is usually quite well-executed by protection and arms specced warriors (with shield). Fury warriors are perhaps the worst I have ever met: 9 out of 10 don't know how to tank but claim they can, allow mobs to hit the main healer while they concentrate on one and whispering to a mage asking why he is not healed.
This is purely for general discussion, feel free to contribute. :-)