Damage per Sec (DPS'sers) - Melee vs. Range-

[Weena2]

Caster dps>melee dps>healers>tanks

heard it here

[Bainz]

As long as they're in melee range, melee can keep up full dps rotation while moving while ranged can only use instant attacks while moving which reduces dps by alot.

Exactly, most Ranged can get around it a bit, the only class that really has trouble are Shamans.
qq.

[Zyrus]

generally the encounter designs softcap the amount of melee you want to bring by adding point blank abilities that would prove fatal if overstacking melee, a trend easily distinguished in parts of coliseum:
beasts - too many melee on the worms -> raid damage that is very hard to handle
jaraxxus - melee having a slight problem dpsing infernals, and too many fires dropping on top of boss can prove to be a problem
champs - not a problem per se, but if you get focused you have a harder time getting away due to not having initial distance
valks - lack of ranged soaking means each wrong coloured orb hitting melee splashes on alot of players at once (not a huge issue with good players)
anub - only real drawback would be that melee is more prone to getting acid debuffs

some examples from ulduar would be thorim chain lightning, snaplashers, mimirons head part, vezax crashes, yogg crushers.
design of encounters generally focus on a requirement for a good blend of both proficiencies

[sebas]

High-end raiders have well geared alts/offspecs especially for fights that require a specific number of a class/spec. This is because you bring the player to the raid, not the class. If you want to raid at such level you need to understand that there has always been favouritism towards various classes/specs on specific encounters and on these encounters you need to min/max so if you want your raid spot you have to put up some extra time in an alt/offspec.

As for Anub, yeah, you need 4 BoPs but on the other hand your raid should be rolling with three paladins to begin with. As for the threat issue, there are various ways to handle the picking up/threat on the adds, depending on your number for rogues/hunters and how high your aoe raid dps is. You don't need 4 rogues in any way. You can do it with 2 rogues/2 hunters or even less if your tanks are on their toes.