Boss HP is proportionally much bigger since Blizzard makes some assumptions about raid buffs, so your individual DPS will have to perform to a much higher caliber.
Group positioning will be much more difficult, especially with things like green ooze on HC Yor'Sahj. Your melee will probably take some time getting used to this, so be patient.
Healers will be stretched thinner, since you'll be running proportionally less (6/25 instead of 3/10).
Because you have more raid cooldowns to work with, raid damage that you could previously ignore will be proportionally higher, so your healers will have to step up their awareness and communication in coordinating damage CDs.
You'll be receiving less tier tokens per member, so you'll have to make sure your comp is split up among the 3 token types evenly. Avoiding running a DK/Druid tank combo, as they'll both be on the same token.
Those are the big ones I can think of off the top of my head. Oh yes, except for the whole finding 15 more competent players thing, which is hands down the hardest of everything I mentioned so far.