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.
I feel like this entire discussion hinges on a key question:
Will our raids be in need of more Healing or more Damage in TBC?
It just seems that about all of the decisions are based around "do we want a healer being less effective to give DPS a larger boost?"
Currently, I would say damage is the most important. Coming into almost every fight, we were rarely limited by our healers gear and their mana pool(consumeables always fixed that anyways). All the fights that we read about that we thought would be extremly "taxing" on our healers, always turned out to be no problem. DPS always saves everybody time, and reduces the amount of time that everybody has to execute their job.
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.
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.