Heroic progress fights are forgiving? Uuuuh...have you ever tried one?
Grinding special items and class stacking do not make a fight actually hard, it is just time investment to actually DO the fight.
On heroic progress today, ALL Cds are crucial and require proper timing. Don't have your personals ready for when your TEAM decides to use them over a raid cooldown, because you have to actually properly cycle/align them to survive many different mechanics? Tough, you're dead. And even on 25 man you only have 3 rezzed, which severely limits your capacity to actually fail and still beat the boss. This is even tougher in 10 man, where u have exactly 1 rez and messing up is that much more painful.
In fact there is very little RNG in today's fights, and a whole crap ton of very noticable and straightforward, but hard to deal with movement mechanics, taking the other elements of a fight that go off simultaneously into account.
I'm sorry to say that like with the rest of the "defenders" of TBC, I highly doubt you have been raiding at a heroic level in MoP or even Cata. Because if you've experienced both on a regular basis you'd think quite differently.