As hinted by GC a couple of weeks back (I love it when I get something right), 10 man and 25 man raids will share a lockout in Cataclysm. In addition, and perhaps just as importantly, the same gear (including ilevel) will drop in 10 man and 25 man raids. I'll look at some of the potential consequences which occur to me:
1) Social consequences
I am a 10 man raider in a 25 man guild, and I raid once or twice a week on my guild's raid off nights. My guild will have to decide whether or not it continues raiding 25 man with mains. We currently run 3 ten man teams on off nights - team 1 is on Rotface hard mode, team 2 (me) has had two evenings of LK training, and team 3 has killed Putricide. Our 25 man raid is on Sindragosa.
Will the hard mode 10 man raiders be willing to wait for the 25 man team to catch up?
Either we'll stop running 25 mans with mains in LK (in which case those people who really love 25 mans will leave), or we'll continue running 25 mans with mains (in which case I might have to leave a guild I've been in for over 4 years if I want to raid). I suspect the former, and we'll become a 10 man guild. I suspect that most guilds except the real hardcore guilds will do the same and the 25 man raid will die away. We've come a long way from Molten Core.
2) Tank balancing
As a 10 man geared tank, I have about 3-5k less health than the guild MT (who also runs 2 lockouts a week). 25 man bosses hit harder because (i) the tank has more health, avoidance, armour and buffs, and (ii) the tank has 2 healers on him.
In Cata, the 10 man tank and the 25 man tank will have the same health, avoidance and armour. In addition, with Blizz chilling out on raid buffs and introducing disposable scrolls, it's likely that kings and fort will be available, meaning the tanks will probably both have the key tanking buffs. What we therefore will see is a weakening of tanks that do 25 man content relative to those that do 10 man content.
Balancing incoming tank damage will therefore be very tricky, since Blizzard won't have as much leeway to increase tank damage from 10 man to 25 man content without the tank getting one-shot. Healing philosophy in Cata is supposed to be that mitigation will matter more because healer mana will be limited, but there is a risk that by upping the tank damage to the level where it can be expected to occupy 2 healers in 25 man content the healers will (unless they have excellent communication) be required to spam heal the tank to keep him alive. On the other hand, if tank damage is not increased sufficiently in going from 10 man to 25 man, then a single tank healer will be able to keep up the tank, leaving a "spare" healer to heal the raid.
Here's a prediction then - "In 25 man Cataclysm content you will need fewer healers than you do in existing LK 25 man content. Healers will need to respec dps to keep their raid spots."