Long list, so brace yourself.
Warrior
The Perfected
If they're tanking, sets a nice brisk pace. Always shockwaves at the right times after gathering up all the mobs, channels the Flash as soon as aggro ever changes, if it ever did in the first place, and will always save your butt, no matter what stupid thing you might be doing at the time. When dpsing, they'll quietly smash mobs until the tank establishes some aggro then toss up their bladestorm or start whirlwinding. They might even throw on a shield to help tank when the tank goes down.
The Defective
If they're tanking, they will be heedlessly charging into mobs into the very back of the group where no healer has gone before. Will probably let the healer die right off the pull if nothing is done about it by the dps. Has never heard of an ability called thunderclap and isn't about to look through his spellbook to find it. Probably has a macro like "/yell "USING SHIELD WALL NOW" and spams it whenever his health drops below 70%. When dpsing, he's already charged into the next group and popped bladestorm before you're done with the group you're fighting, and wondering "Where the heals were".
Death Knight
The Perfected
Knows that death grip is to pull casters in and is able to use it in the middle of a pull to properly group mobs. When tanking, has cooldowns on rotation and doesn't spike because DK tanks don't actually take damage when they're up. Sets a brisk pace, always has D&D down, and rarely has aggro peeled off them. When DPSing, their numbers are always highest in the group while somehow not pulling all the time, throws in a DG to help the tank or simply helps to tank half the time in the first place.
The Defective
As a tank, grips in the closest possible mob because damnit, that's what they want to hit first. Has no concept of threat and instead mashes buttons like he just got out of DK training in hellfire penninsula, might accidentally hit blood boil and misconstrue many numbers as intense threat. As DPS, he's probably already pulled the closest mob anyways and is screaming for you to kill it faster. Pops crit howling blasts before anything gets near your tank, taunts midfight and pops Antimagic shield during Skadi's whirlwind, because he's really not sure what it does. Casts ARMY OF THE DEAD at every opportunity because it makes him feel like a super hero.
Druid
The Perfected
As a tank his rage is never low, he charges fearlessly in to swipe every mob in sight, and probably has rocket boots just because he thinks a bear running at 100mph is pretty cool. Blows his cooldowns every time pressure comes up and understands swipe is terrible, so already has his taunts ready to go as soon as a slight wind peels threat off him. As dps, he is competent and does good damage, unless they are a boomkin in which case the group gets an additional boost from looking at a cute chicken form. As healers they keep their tank fully hotted and the rest of the group topped off, bar none. Might even throw out a cyclone or two or chip in with some hurricanes.
The Defective
Tanking, he's still wearing blues from TBC with +30 stamina gems jammed into the sockets because "he can't find anything better". Doesn't quite comprehend threat mechanics and ends up lacerating everything in sight because he heard it was high threat. DPSing, almost certainly a boomkin who puts insect swarm on everything and then starts casting typhoon because the wave reminds him of the tide. Is probably not even half the tank's dps. As resto he'll probably put a stack of lifeblooms on the tank then start casting nourish. Will be very surprised when the stack falls off and his tank suddenly takes damage and explodes.
Paladin
The Perfected
While tanking, has more hitpoints than god himself and really is impossible to kill with argent defender. Doesn't lose threat to anything anyways, but if he does he's on it ugly like an animal with a hammer or a simple taunt. While healing, he has some ungodly amount of mana that couldn't possibly run dry if he spammed his most expensive spell nonstop for an entire raid encounter. If he's hitting things, his damage makes others aoe spells look weak and measly in comparison.
The Defective
If they're a tank, they've never heard of divine plea before and are constantly drinking like a lush to regen their 5k mana. Will never save your life with a taunt, though it doesn't matter since they're probably not pulling because they're waiting for Avenger's shield to come back up. Healing, they heal with flash of light during pulls of 7 mobs because it's mana efficient, even though they have 30k mana. Have never actually heard of beacon of light. When given a 2hander and told to hit things, explode into aggro fire instantly. Probably even has hand of reckoning as a "dps" spell when all others are down. Definitely has a macro that proclaims "RED BULL GIVES YOU WINGS" whenever they are popped.
Mage
The Perfected
He's one bad dude, but he's probably just using blizzard most of the time because his arcane blasts pull threat like no other. Might throw in a frost nova once in a while to keep dangerous mobs on the tank. Uses iceblock when he's low, and has focus magic on the healer, just because he's a nice guy. He's probably even helping with a sheep once in a while, you probably just don't notice because everything is being AoEd down like usual. Counterspells things casting hellfire, deepfreezes casters, and doesn't just die to things walking towards him at half speed.
The Defective
This is the guy specced 71 points fire who's blastwaving mobs off the edges of cliffs or into other groups because blastwave is a really cool looking spell. Does good damage but rams his face into arcane blasting the wrong mob until it comes over and decapitates him. Has no survival instinct besides backpedalling, blink and iceblock are foreign concepts entirely, not just unused spells. Blizzards as soon as the tank is near mobs and stands in melee range because that's the new black. Occasionally evocates for no reason. If you look at his recount, you'll wonder why Scorch is his #1 damage spell, followed by Ice lance and arcane explosion.
Warlock
The Perfected
This guy is a monster. He'll utterly wreck mobs one by one or just dive in demonstyle and thrash about entire groups. Actually knows where his soulshatter button is, and uses it before mobs chop his face off. Will just cast rain of fire if tank aggro sucks. Throws around a fear, deathcoil or shadowfury or two when the tank is in trouble, or simply to help with pulls. He'll actually use a couple of shards to give you health/soulstones and will sneak in a couple of drain souls when you're not looking so he can do it again and again. You'll wonder why he has a felhunter, but it's probably actually spellocking casters for you without you noticing. Might even be glyphed shadowflame and slowing entire packs for the tank for no reason other than to be useful.
The Defective
Has 70 points in one tree or another but doesn't actually use any of the spells he's specced for. Probably has a felguard and is casting shadowbolt. When he runs out of mana, he's wanding instead of pressing the lifetap button, and drinks religiously between pulls even though he doesn't have to ever. Has points in talents you're not sure why he'd ever need in a heroic dungeon such as demonic resilience, but has never pvped in his life and has no arena team. 80% of his damage is probably corruption, yet he swears he's trying to use haunt as much as possible, though you've never seen him do it.
Rogue
The Perfected
He throws more knives than a Circus performer and destroys groups with ease. If you're tanking you'll love him, since he's using tricks every single time it's up and making sure entire packs stay on you with no effort on your part at all. Kicks spellcasters, stuns vicious melee mobs, blinds that extra add coming into the party, and has probably already sapped something unnecessarily before you ever get there to look at it. His damage isn't too shabby either, despite doing everything else right that he's supposed to be doing, he's still doubling the deathknight who's pounding his face into random buttons. Avoids whirlwinds like the plague, but if they're caught, they know where feint and evasion are.
The Defective
They've never heard of this crazy "Tricks of the trade" ability, but definitely know where FoK is since it makes such big, pretty numbers. Usually die instantly on pulls. Madly stabbing mobs as if they are players in arenas that have to die while that caster over there casts hellfire. Hey, it's ok, he has cloak of shadows if things get really bad. Might use blind to save himself from the mob he's been attacking nonstop, but forgets deadly poison is on and dies to the mob anyways. Pulls groups accidentally and knows where the Vanish button is, but pulls the group back to you first so they aggro and kill someone before vanishing. Don't worry though, he's got the cheap shot opener.
Hunter
The Perfected
Volleys like a champion, but you might notice a silencing shot on one of those mobs that's been misdirected towards you, or the mobs running really slowly over a freezing trap. You might even notice that out of the way mob with in a frost trap, but the hunter is still shooting the mobs? Misdirects every time it's up and does some beastly damage. If he's ever in trouble, deterrence is popped right away or he's already 30 yards away, shooting the mob with a scatter shot. That is, if you even noticed, because he probably already feigned before the mob went anywhere in the first place.
The Defective
Never actually used feign death levelling up because he specced every point into beast mastery and his pet held so much threat. Constantly pulls large mobs and bosses laying into them without a misdirect, and pops deterrence instead of feign death. Feign seems to be "resisted" a lot, according to him, when really it was never used in the first place. Volleys randomly in strange situations, where 1 or two mobs are together, and uses multishot on single mobs as part of his rotation. Damage is almost certainly 70% autoshot, because he is afk eating ramen or browsing the hunter forums on EJ instead of being in the instance. Has never heard of any crowd control other than killing the mob.
Shaman
The Perfected
This guy is buffing your group, casting bloodlust at the right times, and tearing through faces with his lava bursts or spirit wolves. Probably the best player in your group in disguise, he'll quietly do his job and cut down mobs with no remorse. You might even see a frog or two running around just because he feels like being useful. Tremor and cleansing totem are always down, even if his damage is suffering a bit as a result, you'll never care. If he's healing, even the most brutal of group damage is pretty meaningless as he shotguns off chainheals like it's no one's business. Actually knows when and where to use his elemental totems to help with certain fights. Can probably heal you just fine as elemental while outdamaging most of your party.
The Defective
Flailing inefficiently with 2 1.4 speed daggers, the defective shaman is probably pulling about 800 dps, mainly because he has no clue what his class actually does. His weapons have double frostbrand on them because he thinks the colder the enemy is, the less effective it'll be. Might've specced elemental because he thinks lavaburst is really cool and has a very large number when it hits, but doesn't cast anything else, and is probably afk most of the time. Has never heard of spirit wolves. If he's healing, he'll have a bunch of totems that give buffs, but none that actually negate terrible ailments which will cause your party to wipe. Continually heals with Healing Wave because it heals for a lot, but lets group members die because he hasn't talented for it and casts nothing else. Never used riptide once in his life.
Priest
The Perfected
This guy is a healing machine. Whether he's disc or holy he's throwing out shields, making sure guardian spirit goes out each time it's needed, and even shackling those pesky undead. Might even throw up a couple of dots or Mindcontrol a mob after shields and renews are up on the tank, just to get the encounters out of the way quicker. Tosses on holy fire when he can simply because it looks awesome. If he manages to die, he'll sit in angel form for as long as humanly possible before apologizing to the group. If he's dpsing, his mindsear brings all the boys to the yard because that's right, it's better than yours. If somehow he manages to pull aggro he's all over fade or dispersion like bees on honey.
The Defective
Probably played a paladin and blows through mana like water casting spells that aren't needed. Uses flash heal exclusively even in large pulls and is always confused why the tank died even though he was "casting nonstop". Has never heard of Hymn of Hope or shadowfiend and will never ever burden himself with the knowledge. Thinks Circle of healing is enough to heal large aoe pulls and is constantly wondering why people died, surely the damage wasn't that high. If he's dpsing, his damage is abysmal or amazing, but has no idea where the fade button is and dies without that knowledge.