The Stress Factor:
Technically, it's not about the office per se. It's the people in it, and everything they do -- whether you realize it or not. The sneaky thing about stress is that you can feel it without knowing exactly what's causing it. There's the specific kind of stress you get from a guy chasing you with a chainsaw, and then there's the more subtle form caused by the little things -- like a conversation going on nearby that you can't quite hear, or the squeaky wheel on somebody's chair, or somebody continuously walking back and forth behind you. Open plan offices are full of that kind of stuff. You can't put your finger on it, you just involuntarily feel your hand clenching into a fist and think how nice it would be to just punch a dude through a wall.
Or several easily assembled walls.
In one research project, they made a guy work in an open plan office while wearing a helmet that measured his brain activity throughout the day. They found that, despite the guy getting to spend the day pretending he was Professor X and that his desk was Cerebro, his brain was nervously working on overdrive all the damn time. It was processing all of the disturbances in the office, and we do mean all of them -- the subject wasn't just flipping out at the horsey laugh of the dude at the water cooler. Instead, his subconscious took in every little thing around him and rolled it into one giant ball of "I've had enough of this shit."
"WHY THE FUCK DOES THIS THING KEEP RINGING?!"