A couple months ago I was one of those huntard players who stunk out the joint with not too good DPS, gaining the mockery of people in PUGs who ran recount to see the woeful numbers. Yes, back then, I was pulling 700-900 dps, running heroics. Not a good fit. How did I get to that point? What made me wake up to playing my class (In this case hunter), better?
I posted last week about using the LFG tool to run PUGs to learn how to play your class, and deal with group dynamics in instances. When I was levelling the hunter, that tool didn't exist. Most of my instance runs were soloing vanilla content when I reached a high enough level, or helping guildmates leveling who need a hand with Stormwind Stockade when you're level 76... When you run instances like that, you're not exactly learning how to play your class. That becomes a problem when you hit 80, have unenchanted/gemmed gear, use vendor ammo instead of ammo with more damage you can get off engineers, increase inappropriate stats... You get the idea. It was pretty bad. As a hunter, I should have focused on agility as the main stat. Instead, I focused on stamina, because I saw it added hit points to my health (which is a secondary stat), and strength. It was messy, and nobody noticed it. If they did, they didn't bother to point it out.
I was able to get away with poor DPS on normal instances using the LFG tool. On heroics though, I learned a hard lesson. You WILL get called out if recount is showing DPS numbers that another member of the party deems too low. After several times of this happening, I decided to finally learn about my class. I studied up on shot rotation, overhauled my talent tree, got advice from other players, got purple ammo, and am currently working on getting more appropriate gear for the hunter (a slow process to be sure, but getting there).
The results are paying off. I still have some work to do, but I'm not getting called out low DPS anymore. The key thing is to take time to learn about your class. Happily questing along by yourself is a different cry from running instances and raids with other people. There's an expectation at that point that you have at least a general idea to play your class. Take the time to use LFG, especially at lower levels, go to sites such as Elitist Jerks, and seek out the opinions of more seasoned players. You'll be better off for it.