Open plan offices are often sold as collaborative workspaces. However, without proper planning, they are little more than a cost saving measure that does nothing to foster collaboration. An open space with people seated shoulder to shoulder without thought given to layout or how the inhabitants might work together just becomes a noisy and distracting place to be. This is not the sort of environment where people can be productive and collaborate.
This is not to say that all open plan offices are bad places to work. Some organizations design their spaces with collaboration in mind. Aarron Walter describes the thought process behind designing the MailChimp design studio–there are some excellent ideas there. He also recommends a book, Make Space, on the subject.