One thing I question is just how 10 man hard modes are going to be just as hard as 25 man. It seems the only way it can be done is to tune it so tightly that you HAVE to stack the group properly. This tends to clash with the "Bring the player, not the class" mantra. Now, they are homogenizing some more with Cataclysm, but if fights are VERY tightly tuned, you're going to want to stack buffs as well as you can to get through them.
On a separate note, I hope that "hard mode" in Cataclysm means more than more health, more damage, more healing. Firefighter has always been a fight that stuck out in my head that really made the fight different. I had high hopes in spite of ToGC that ICC would hold real heroic modes. I think they struck gold with the Lich King fight, Putricide is not TERRIBLY different, but it is quite fun and still adds a new layer of difficulty. Other than those two, Blood Prince Council and Saurfang add a thin layer of difficulty, but they are not substantially different by any means. Otherwise, the changes are so minuscule (or even nonexistent) in terms of fight mechanic changes that it's really just the same fight tuned for better gear.