Constructing a raid group is all about synergy, understanding and trust. For success, you really need to ensure that your 10 or 25 people are adding up to more than the sum of their parts and that they will act or react in a way that is considerate to the rest of the group. In most efforts to cover this topic, commentators will attempt to follow this statement with either a dressing down or a staunch defence of how important "output" is. Bearing in mind "output" is something of a subjective term, it's hard to establish just how important it is when variables throughout an encounter will considerably skew it one way or another. What I find frustrating is that a lot of the discussion devolves into the value of damage meters; even more frustrating is that many people seem to make terrible inferences from a very shaky grasp of logic.
1) If you like damage meters, you like spamming them in party and judging everyone's ability based upon them.
2) If you don't like damage meters, you're a casual baddie who isn't interested in improving your own performance.
Alas, human duality (and stupidity?) precludes people from getting beyond these two extremes and assessing the value of damage meters more objectively. I like damage meters, hate judgements based purely on them and am very interested in performance improvement. But in this world of "output" and raid-wide symbiosis, damage meters and parses are becoming an everyday part of the raiding experience. Whether you like them or not, their influence grows with each passing day. If you want to be a raider of progression content, it's best you accept that someone is going to be poring over what you're doing and trying to judge your performance accordingly.
Context here, is key - I've just come off the back of a raid last night that was, with no exaggeration, one of the most enjoyable I've had in a long time. We welcomed a new member into the fold, put together pretty much our strongest group, the composition in the raid was complementary and the person I asked to sit out for the night did so cheerily and with no argument. When you add that to a one-shotting of every boss we went for (heroic Halfus excepted - I always balls up a couple of pulls) and the inclusion of an achievement we didn't have before, we all had an absolute blast and can't wait to raid again tonight.
A lot of people believe there is more value in a post-mortem after a bad raid night, and there is value in that opinion. When things are going badly, it's worth knowing who's knuckling down and, conversely, who's throwing in the towel. But when things have gone well, you get a glimpse of that hard-to-appreciate concept that every guild should be looking for, particularly when starting out:
Last night, I saw a bucket load of potential. Not only in performance, but in attitude and raid... Togetherness. Last night was people playing for one another in a complementary fashion that saw everyone's "output" increase. Yes, our new warlock is now indoctrinated with the "Blame Zell" mantra; that's unfortunate. But we broke several of our own records, passed out some nice loot, got a new achievement and even recovered from three deaths against the Ascendant Council to one-shot it. Oh, and our warlock was new (slightly lower geared than most of us), while our Shadow priest was in a dud spec that put her way under the hit cap. The point it's worth coming on to is whether or not "output" matters in such an environment. The thing is, that's actually a bum question.
Last night went so well BECAUSE the output was generally high across the board.
Damage meters and parses are invariably used to haul people over performance related coals for no actual benefit. If the tank threat was a bit lousy, healing was slack or DPS was low, then I'm confident there was a reason other than laziness and I don't actually need a meter to tell me that. The way we do certain encounters doesn't necessarily help certain specs and, in some cases, will hinder them. Our Feral's DPS is always low on heroic Atramedes because he never spends any time behind the boss. Our Retribution paladin never does high damage on Al'Akir because he coordinates the stormling strategy. Our aforementioned Shadow priest often falls behind due to a willingness to throw out heals or go the extra mile in avoiding damage. And no matter who I ask to do a job that will impact their DPS negatively, they're always happy to do it because they're far more interested in raid DPS than their own.
The key to all this, and where meters play the biggest part, is in each player's self-evaluation. As Kripparrian likes to say, you shouldn't necessarily set a number on someone's output; "you need at least 12k for Cho'gall" can easily cause someone to get there and start slacking off. You should be turning up to every single raid, determined to do more damage or more healing than you did the week before. Your meter can tell you if you're improving your uptime, getting in more globals, choosing your abilities better or taking fuller advantage of your procs. Each week, you should be fighting against yourself and not necessarily the rest of your raid because an encounter simply may not be made for you, making cross-class comparison totally pointless. You're not just fighting to beat enrage timers (be they soft or hard), you're fighting to make the encounter easier, and thus more enjoyable, for everyone. The Raging Spirits when fighting Arthas played no part in the enrage timer, but the difference made to everyone by high DPS finishing them quickly was glaringly obvious.
Gear, spec, encounter specific tasks, latency and raid composition all mean that a straight one-for-one comparison has no value. Instead, use your meter to maximise your OWN performance and you'll start to see across the board improvement in "output", enjoyment and success.
If raiding had a holy trinity, that'd be it.
Output, enjoyment and success.