A shared space is fine with <5 people, can become problematic at 5-10 people and the frequently 20+ large floor spaces are just unworkable without noise cancelling headphones.

I worked in london in an open plan office maybe 40ft wide and getting on 200 ft long. That's open plan.

It's fine except for the noise. It's always the noise as the problem, all else is ok.

(edit: not a single company, a collection of small and medium businesses)

And the utter lack of privacy. Want to tell your colleague that Alex is a dick? Better do it in another building, or the entire office will hear it.

The other problem is people trying to do Skype or other conferencing calls while wearing closed-can headphones. Their own speech level rises two notches when they can't hear their own voice. Try it for yourself and see.

Open office seems to be designed for people who like to yell at each other or who isolate themselves completely via headphones. I have almost never seen any meaningful collaboration in large open office. It always happens in smaller more intimate spaces.