If you don't want all the pointless context, you can skip the first few paragraphs. I know I would. Who wants to hear about lowbie randoms?
My little alt paladin is 35 now. That means he can queue for SM cath. But when mobs are 3-4 levels higher, pulling must be done carefully.
Let's backtrack a bit to level 33, or maybe 34. I'd queued for a random and gotten SM lib. The healer quickly proved herself to be capable when we started the first hallways and runner after runner meant that I was tanking 5-6 mobs at a time pretty much down the entire hall, as one group got low it would aggro the next, and so on. No time for rest. In contrast the warlock was absolutely terrible. I'm not sure if he actually casted anything and his blueberry was poorly controlled, constantly aggroing new groups even when there wasn't a runner. At the end the fury warrior rolled need on the caster dagger (against the good druid healer), refused to trade it until we explained why he didn't get a cache of random goods, and then left after saying "what dagger?" The druid had a rogue guildy with her.
We did another random, getting SM GY. This was when uselessness set in. Not for me, but for pretty much everyone else. At 35 the mobs are green and I can easily pull until dazed, drop consecrate, and do loads of damage. Meanwhile the rogue is stabbing one at a time, the warlock is already useless, and I forgot what else we had. That's a rather unfun situation, to be useless like that. I felt sorry for them but what was I to do? I spotted a rarespawn and the warlock got a hat. The druid saw the offhand drop for what was apparently her second time, we noted that it would have gone perfectly with the dagger she didn't have.
Onward to specifically queuing for SM armory. Trash trash trash trash. The healer said I was a good tank. I explained my tanking policy: The mobs may only hit me or DPS who I don't like. I enforce this rule very strictly. Then Herod gave me shoulders that I can't wear yet.
Then we went to RFD. We kicked the warlock after he continued to be silently useless (I hate it when people who don't respond at all in chat) and rolled need on some mail. A mage joined. I got a plate chest. Someone started the escort too soon and two people didn't get it, including the mage. He left. Someone joined. They left. Someone else left. I leveled up just in time to have enough mana to grab the last wave and boss. Then we carried on with no problems. The druid got gloves and the rogue got a dagger and shoulders.
Onward to SM cath. It went fairly smoothly until the third pull in the cathedral. Okay before that the shaman annoyed me by demanding that we pull the other side too, after I'd just pulled a mob from that side. I wasn't watching the pats carefully and got two wizards (they are so OP) and some melee mobs. I died. The group died. The mobs reset. We ran back and the level 40 warrior in all manner of heirlooms asked if she could tank instead.
I said that I had no DPS spec, so I'd just be uselessly leeching XP. The rogue supported me. We kept running to the cathedral while the warrior explained that she could take a lot more damage than me and hold aggro perfectly. Someone initiated a vote kick on her. I whispered the rogue "she's just trying to help". Which I thought she was and she had a point after all. The kick went through and the shaman didn't requeue us because it's more XP. That seemed dumb. We lost so much DPS that the XP gain was offset by slower kills.
I didn't want to feel useless, auto-attacking with a 2h with 10 second judgement, low crit, and no 2h talents. It's a terrible feeling. It's the feeling my priest has gotten in some very fast heroics where I barely get off a mindflay or dot and the mob is dead. It sounds weird to say "My DPS would be higher if yours was lower", but it's true.
There's some maximum possible DPS in a heroic. I don't mean from gear or skill, just that mobs only have so much health and are spread out. If a mob dies in five seconds and your spells are 2 second casts, you're getting off three attacks maximum. More likely you're a caster lagging behind slightly and you get two casts. Right there, a 33% reduction in DPS.
Some classes are better at trash that others. Paladins are great with seal of cleave, a lot of instant attacks, and an AoE attack as part of their normal rotation. Warriors work well too. Then there are the rogues who need to get behind the target to use their best openers and build combo points for the big attacks, but the mob might be dead already, so the points are wasted. I don't imagine kitties are in any better of a position. But those two have it easy compared to an affliction warlock. Poor dot spec. This is one benefit to destruction: nukes.
Trash meters are little more than an indicator of what class the person plays. Highest? Paladin. Lowest? Rogue or warlock. Only boss meters can demonstrate much about player skill. And yet how often are people kicked before the boss, or kicked due to overall DPS? It's ridiculous. When I've looked at my rogue, trash is pitiful, 1500 maybe, while bosses are minimum 2k, but more likely 2.5k. Not great, but surely above the threshold of terrible. If only trash lasted long enough to get a finisher off!
Next time you think you're carrying people in a heroic, consider that they might not be asking to be carried. They might be little kids walking a little slow, but at no point did they just sit on the ground or whine "up up up!" You just picked them up without asking and then complained about having to do it.