Over my term in WoW I've filled the role of MT/Raid Lead, raid/MT Healer, and melee DPS. I dabbled a few times with ranged DPS, but I never quite got into it. From MC to Ulduar, with all the raids in between - there's a few lessons I've had to learn the hard way. So in no particular order - I give you WoW's lessons from the streets.
Don't outpace your healers.
Chain pulling is all well and good, but your healers are quick to let you know when you've overstepped your bounds. It may be a simple request for mana, or letting you dip a bit - or it can go so far as to warning the OT that you're about to die. Also, don't leave them behind by charging in at the mob. God (Chris Metzan) gave you a ranged pull or mis-direct for a reason. Use it.
LoS is serious business.
I will never forget (partially because the healer won't let me) the 3v3 Arena match where I learned this lesson so well. It was in the Nagrand Arena (I think - the one with the 4 pillars) and it was our pally/mage/war combo (I was the war) against two trinket/PoM mages and something else. I just remember charging at the mage, getting pillar humped, and hearing Toby cry 'NOOOOOOOO' as I ate two trinketed pyro-blasts and was insta-gibbed. Toby and Frog managed to pull that one off somehow - but I spent a lot of time looking at the floor.
Nobody really means it when they say stand in the fire, except of course when they really do.
During an attempt to teach SSC to a former guild, we were clearing out to Leo. I saw numerous raid members standing in the rain of fire - taking significant ammounts of damage. I piped up on vent - "Don't bother moving out of the fire - the healers need the practice". One hunter was ballsy enough to actually ask 'Really?' - he claims he was being asked by other raiders, I think he was just being dumb (<3 to you Chris). I let him have both barrels on vent. It was pretty funny.
Of course when a wipe is called, we really DO want you to stand in the fire. Yeah, it's confusing if you're three. Deal with it.
Pugs make you better.
Really. Think running heroics is too easy with your normal crew? Hop in LFG and heal/tank/dps the first thing that comes along. I guarantee you'll use more cooldowns than you have in a month, and you'll finally find a use for the poultrygizer.
Stealing this one from Tobias - like your job.
Dude told me today - if I've taught you nothing else, I hope I taught you that you have to love your job. Because nothing is sweeter than pulling off a near wipe and having the tanks let you know they felt it. You have to be humble of course - but you know what saved the day.
Remember that quest on Theramore where you have to bandage the wounded soldiers before they die? That's raid healing - and knowing that someone is going to have to bite the big one on a given pull is just part of the game. It's ok to let someone die. Especially if they are Nickleback fans.
Finally - Have a DPSer somewhere.
Doesn't matter what server, class, or spec. Have a face melter in the shadows that you can decompress with. Nothing helps relieve the tension of a night of wipes like smashing faces in PvP/Arenas. Just don't take it too serious.