It was just the mechanics of fights or the game system design itself that made Naxx 40 as brutal as it was. Sapphiron needed a boatload of Frost Resistance. Razuvious needed spell hit capped Priests. Four Horsemen needed 8 (yes, eight) tanks to do it. Loatheb's DPS check was insane as well as healing allowed except on MT. Heigan and the safety dance wasn't too bad, although as adam states above any blip of lag could kill you, but trash before him was even worse. Thaddius was Maiden of Vigilance on crack and super buggy, I guarantee at least 5 people missed the jump to his platform every try. Gothik needed about 6 Priests for shackle or just don't even bother trying. Mind control resist on Razuvious? probable wipe, Taunt resist on 4 Horsemen? wipe, Hunter too slow/fast with Cheetah on Anub'rekhan? wipe, 30 minutes gone by since you started a boss? time to reclear all the trash.
If they are using the original mechanics and system it should be just as hard, even with everyone on amazing internet connections and PC's.