I have no idea how a 757 would react after taking off with out the flaps set (forgotten) in the T/O position, but I would assume it would be sluggish and in real danger of stalling. Perhaps Lou can inform us.

I would like it if the next 757 (5.0) would react similar if the flaps are not set for take off, in that it will take a lot more runway and if you attempt take off.... you won't get very far.