I am a raid leader of a 25-man raid group currently working on heroic blackwing descent. (I am a tank myself)
As long as I know, most raid bosses in Cataclysm prefer ranged DPS than melee. Having more ranged DPS will effectively lower the difficulty of encounters.
Melee DPS often need to move around more, find the back of the boss to avoid parry, and in charge of spell interrupts (which was very common in Cata raids). They could do similar DPS as ranged classes on single target, but they just don't stand a chance on massive AoE encounters, either because they can't do enough damage (think of Heroic Maloriak) or the encounter simply does not allow them to AoE (think of Cho'gall or Heroic Conclave). They often suffer more damage and are more likely to die early. (think of Ascendant Council phase 3).
Just simply list the disadvantages melee DPS have against ranged during encounters.
Conclave of Wind - Cyclones near the East Lord, more time to run to the bridge than ranged, not able to AoE adds on West Lord (Heroic)
Al'akir - Not able to DPS the adds in Phase 2 due to aura damage; may have to hit on the face
Halfus Wyrmbreaker - The inferno animation stacks near the melee range, making it harder to dodge
Valiona and Theralion - No specific disadvantages but some of the ranged class could do DPS to both dragon with DoTs
Ascendant Council - Excessive movement during Phase 2, need to suffer from the ice damage beneath the boss or run away to avoid it, lowering DPS
Cho'gall - Not able to AOE the Black ooze due to corruption bar
Magmaw - No specific disadvantages on normal but I heard that there are some on heroic
Omnotron defense system - May be not able to DPS the green oozes if they fixate on melee; may have to hit on the face when the boss is in the green cloud
Maloriak - The most complex interrupt fight ever. And they can't do enough AOE damage on heroic
Atramedes - The sonic circle came out just beneath the boss, making melee harder dodging it.
Chimaeron - No specific disadvantages but they often die faster on phase 2
Nefarian - Not able to DPS Nefarian during Phase 2
It turns out that if you just minimize the number of melee classes, the raid will be easier than using a more balanced group.
The ranged class often have raid utilities (add snares, multi-targeted DPS), move less in an encounter, and are less likely to die. (Maybe they stand close to healers)
This is just not right, Blizz should compensate the melee classes in terms of DPS, to make raid leaders more willing to bring melee DPS to encounters.