OK, let's look at loot distribution.
Badges will be converted into a points scheme that pays a little more if you do the 25 man raid. You'll be able to supplement your points income by doing 5-mans so characters that can tank or heal are advantaged. On the other hand since you can get points by doing 5-mans there's no reason to prefer 25s for points.
Tier sets are by class. So in a 10 man raid you can have perfect distribution with 10 players, one of each class.
Standard loot table items have traditionally been split fairly evenly along the following lines:
See there are 9 possibilities and generally speaking the loot tables aim for one of each for every boss. This is a huge advantage to a raid that has the spread of loot type users over a guild with, say, a lot of guys in plate doing melee and a lot of cloth casters nuking and healing. (for an example see Valithria's drops at the end of this WoWwiki article).
Let's now look at the Cataclysm specs that correspond to those armour types:
Tank plate Prot Warrior, Blood DK, Prot Pally, (3 specs)
Dps plate Ret Pally, Arms Warrior, Fury Warrior, Frost DK, Unholy DK (5 specs)
Spellpower plate Holy Pally (1 spec)
Dps mail Hunter, Enh Shaman (4 specs)
Spell mail Resto Shaman, Ele Shaman (2 specs)
Dps leather Rogue, Feral Druid (4 specs)
Spell leather Boomkin, Tree (2 specs)
Nuking cloth Warlock, Mage, Shadow Priest (7 specs)
Healing cloth Discipline Priest, Holy Priest (2 specs)
Now there are two possibilities with raid composition. Either a raid guild recruits by armour type in which case it's hard to get in if you are in one of the popular categories or a raid recruits whoever in which case people in the popular categories probably have to share loot with several other people.
Best thing to be for loot: Holy paladin
Second best: Healing Priest, Boomkin, Tree, Casting shaman
Second worst: DPS Plate
Worst: Nuking cloth (7 times less likely to win an item for your role than a holy paladin)
There is a minor advantage to being high up the tree. A casting shaman could pick up caster leather or cloth, a holy paladin could pick up cloth, leather or mail in addition to their expected loot. This advantage has specifically been removed for DPS Plate who will be Strength based while those in lighter armour will be Agility based.
There is a workaround to this - if you cherry pick your raid you can eliminate anyone who rolls on the same loot. I saw this happen in a pug raid in WotLKwhere the raid leader decided the 25 man team was full on Death Knights at one DK and turned away several during the 20 minutes it took to form the raid. I wonder if he'll practice the same method in Cataclysm - turning away 1 player every 6 recruits.
Long term that workaround won't work, you'll just force other plate dps on your server to do the same thing and exclude you rather than let you form raids to exclude them.
People who are in a minority armour type in a 25 man raid will do best. A holy paladin or a caster shaman may be the only person in his armour type category and so can expect 1 in 9 drops to be for him.
1 in 9 could mean a lot of epics very fast. If a 10 man boss drops 2 pieces of armour and a 25 man boss drops three times as much to split between 2.5 times as many players that means that if you're the only caster shaman or holy pally in the raid you see 2 pieces of armour for you every 3 bosses.
In 10 man raids it will be very common to have sub-optimal distribution. You may be in a raid with 5 clothies that disenchants all the spellpower plate and mail. This will slow down your raid progression.
Where you have optimised distribution you will have one player over. 9 armour types and 10 spots. So it's likely there will be a couple of people who gear up significantly slower than everyone else, eg the mage and the warlock.
Offspec loot will be extremely hard to get if your offspec is in a popular category. In other words healing priests won't be able to get caster cloth, healing paladins won't be able to get ret gear from raid drops very easily. On the other hand a caster offspec for a feral druid, ret pally or enhancement shaman will be very easy to gear.
I will play either a boomkin/tree dual class druid or a resto/elemental dual class shaman in Cataclysm. I think the Shaman will be optimal for loot, especially if I play Alliance. It's the most unpopular class already and there's a fair chance that some of the people who dropped their previous main to roll Draenai will drop their shaman to play a Worgen. A lot of people will roll Druids, especially Worgen players.
On the other hand I do rather like Druids.
If I lead or help lead a raid guild I will urge my guild to recruit by armour type and maintain a hard cap of 2 in each armour category and in each class. This is quite harsh - if we have one mage and one warlock on our roster we will be full on cloth nukers. Sorry shadow priests!