If all you did was look at this chart:
http://raidbots.com/dpsbot/Overall_D...10100000000000 (10m Normal parses, best spec for each class only) You could pretty much come to the conclusion that affliction warlocks were way OP and that melee (except for enhance shamans and rogues) are crap.
However overall charts are probably fairly biased because you get one good/bad fight and your numbers go out of whack.
If you click through all the individual encounters you notice that the classes jump all around. Sha of Pride for example Warriors and Rogues top the charts. On Thok, hunters take the lead.
You also have to know the individual fights to know -why- the numbers look like they do. Hunters on Thok are the only class that can ignore the big aoe interrupt during phase 1 and continue to DPS the boss with their full rotation during the chase phase 2.
And sometimes the numbers are skewed. Fire mages look REALLY good on Klaxxi, but that's because they're damaging all three bosses. They're doing this to increase their single target damage to the main target, but the splash damage adds up, even though damage done to the off-targets is worthless in terms of actually winning the encounter.
Are there some trends of underperforming/overperforming classes? Of course. But its hard to just look at charts and say "Gee Arms warriors are really really bad" without fully understanding the class and how it works and how it interacts with the fights, and what it brings to the raid.