Maybe for the normal versions.
Some of the guilds on the PTR are pretty advanced guilds worldwide, so things that they one-shot might give other ppl trouble. So PTR only helps Blizzard that much.
Maybe towards the later bosses they overdo damage a bit to give the great mass of ppl more time in the Instance. One shotting everything isn't really fun.
One boss per night is a bit better.
If ICC is like Naxx (in the way that it has 4 quarters and then on to Arthas), 6 weeks for an average guild to get to the last boss sounds good to me....but I am not in Blizzards brain or mindset.
Ulduar difficulty would be nice. And I wouldn't mind Arthas being like Alagalon in a way. Big cockblock to see. For all I care they can remove the obstacles to him a month before cataclysm is released to give "everybody who cares" the chance to kill Arthas.
But I won't die if Normal mode means everybody beats Arthas in 2 weeks. Plus it means I have a reason to post on this forum for the next 4 months about the huge QQ shitstorm how this game is too casual