I'll limit my response to just which buffs I'd prioritize, as I think this starts to get into the whole "what's the optimal 25 man raid" question that's been heaped a few times.

If you're trying to prioritize melee buffs, I'd say totems, then battleshout, anything else is nice to have. Battleshout works out to a ~20-25% increase to AP for rogues which is a significant chunk of dps. In a pinch that warrior can also boost everyone in his group's health by 730. Since it's either commanding shout or battleshout for warriors now, you probably will not ever want a rogue in with the MTs.

I can see paladins in the MT group, but not the melee dps group. Shamans seem much better suited to supporting dps, paladins are better for defense. If mitigation really matters the shaman can always drop a grace of air totem which will bost the AC, dodge, crit rate, and AP (for the rogues) of their group. I'd much rather have that over the higher AC of devotion aura. For rogues, avoidance is usually more important than armor mitigation since we'll usually get 2 shot without a heal even with a higher AC.

Effects like Imp LOTP are more supplemental healing than replacement healing. In most encounters where there's splash damage, you're taking more than 4% of your health every 6 seconds. It's nice to have, but not game-changing. The main reason to put the druid in there is to maximize Unleashed rage uptime, flurry uptime, and boost your rogue's dps by from the higher crit rate (seal fate and lethality will increase this beyond the 5% you'd expect).

If we're talking just pure dps, you'd be better served by having an enhancement shaman, fury warrior, 2 rogues and either either a rogue, druid, or (less likely) hunter. The question is whether or not the dps lost from the rogue who isn't getting the shaman/warrior buffs is greater than the gain the other guys get from having a druid or hunter.

TSA is a static amount of AP, 5% crit from LOTP and windfury totems scale a bit more quickly with gear. It will be a function of spec, encounter design, and gear as to which buff is "better".

Tank group (1)-
In this group would go the MT, a DPSadin (Sanctity Aura), Tree Druid if it's there, the OT, and either a Holy Paladin (if the MT or OT isn't a Paladin with Devotion Aura) or an encounter-specific class. I'm not including Shaman in there because they have huge DPS buffs (totems and Blood Lust).

Physical DPS groups (2)-
Just stick all the Feral Druids, Enhancement Shaman, Warriors, Hunters, and Rogues in here. One max Feral Druid and Enhancement Shaman per a group, to avoid overlap.

Caster DPS groups (1.5)-
How to divide up caster DPS group buffs depends a lot on the encounter. Shadow Priests and Shaman division is dependent on how heavy the mana drain is on the healing vs DPS.

Healer group (0.5)-
This group slots in with the caster DPS group. The group buffs overlap a bit.

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The Shadow Priest is probably the hardest character to slot into a group. The group aura they provide is insanely powerful. Of any spec in TBC, Shadow Priest is the one I expect groups to use the most of. If Vampiric Touch continues to stack, I'd expect groups to seriously consider taking three Priests for a raid. Even if it doesn't stack, I'd always shoot for one Shadow Priest for each of the caster groups. Of course, the odd thing about it is that the spec that benefits the most from having a Shadow Priest (Elemental Shaman and Balance Druid) offers the least to the Priest because of the weakness of crit for the Shadow spec.

Just saying "You want one of everything" doesn't work when you put it into a grouped setting. Sure you want all of the buffs possible for maximum effectiveness, but you still require a certain number of people performing a certain number of primary roles and not just being along to buff others while under-performing in a primary role.

The following group of:
Fury Warrior, Feral Druid (Imp.LotP), Enhancement Shaman, Hunter, Rogue

Looks great, but does it perform the role of Self-sustaining DPS better than:
Fury Warrior, Rogue, Rogue, Rogue, Resto Shaman

or
Fury Warrior, Rogue, Rogue, Feral Druid, Resto Shaman


Where do the off-spec buffs to the primary roles outweigh just another person performing their primary role?

When they buff overall raid dps? I'm guessing you're looking for some kind of math here; but the situation is dependant on about a billion variables. We can venture a guess that in most every situation for dps:

2 rogues, 1 feral druid > 3 rogues > 3 feral druids

Same goes for enhancement shaman. If you're talking about total raid composition, then 1 of every type, 2-3 of every class is a good overall plan(notice that you still are including non-dps-buffing dps-only classes; 2-3 rogues, 2-3 warlocks, 2-3 mages). If you're talking about group composition, yes, ideally you mix in the pure classes with the party-buffing hybrids.

And therein lies the judgement call that will contribute to making tBC *more* fun than WoW 1.0.

Take for example feral druids. I've played with good ones, and I've played with shitty ones. There are types of encounters bear tanks are good for - and math can tell you when to bring a kitty along. However, over and above the "what type of encounter is this" question, you have to ask yourself, do my feral druids suck? Are they geared appropriately? This is the gray area that appears to be causing some sort of block. This is one of those things, unless I'm misinterpreting the question, that there is no answer for.

Know your guildmates, know the encounters, work with what you have. Which, were I given my druthers, is how I'd prefer the game operates. I believe a lot of readers here would agree with that sentiment - the "You need X raid comp for Y encounter" business has gotten stale.

Another consideration that a lot of people won't realize: Beastmaster specced Hunter pets get at least as much of a benefit (in raw DPS terms) from the melee group layouts seen here as rogues do, plus the hunter gets some minimal benefit as well and provides a 3% damage buff to the whole party. I think that war + sham + hunt + feral + x will be an ideal group, as long as the hunter is bm specced. MM specced hunters would probably be best in a mixed group.

With the reduction to 25 mans, group composition will be much more dependent on what you run with -- the difference between running with 9 or 11 healers out of 25 is much greater than the difference between running with 13 or 15 out of 40.

Generally speaking I see there being a MT group (of course), 2 DPS groups and 2 caster groups. Your hybrids will fall into those groups as their talents dictate. Shamans and Paladins will fill out the auras/totems where they are most needed. I would priortize DPS buffs over anything else, with the exception of one shadow priest for a caster group -- i.e. I would be getting those shamans in the DPS groups for totems. Imp LOTP would be in a DPS group -- not so much for the self healing (which is very nice of course) but the added crit is a very nice bonus.

The shadow priest would probably be the one switching between the caster groups as needed. If it's taxing on the healing, then I would place the spriest there to be a mana battery for the healers. Otherwise, with the warlocks and lucky mages.

Going any further in depth on this is again, quite specific on exactly what you're bringing to your particular 25 man group. I think the theory I would continue to stick with is "DPS is king" though.

Nope, really nothing productive here.

Basically this is like asking "what is the best class"? It's a meaningless discussion without specific parameters, and we lack all those parameters. Anyone who reads this board hopefully knows that a shadow priest should go in a group with mana users and that a LotP druid would do better with melee than in a group of mages. Beyond that, group composition will depend on the specific scenarios, as it always has.