"Brainstorming certainly looks like a great way of dealing with some of the problems associated with decision-making and creativity in groups (...) By suspending evaluation, encouraging a relaxed atmosphere and quantity over quality, the brainstorming session is supposed to foster creativity.
Experiment(s) show that people in brainstorming sessions produce fewer and lower quality ideas than those working alone. Examples:
Social loafing: people slack off to a frightening degree in certain types of group situations like brainstorming.
Evaluation apprehension: although evaluation isn't allowed in a traditional brainstorming session, everyone knows others are scrutinising their input.
Production blocking: while one person is talking the others have to wait. They then forget or dismiss their ideas, which consequently never see the light of day."
Growing up I always thought that brainstorming helped produce strong results. Last semester, one of my professors told me that he was actually against brainstorming in large numbers. He beileved that a larger group of people would have to settle on a safer more agreeable idea. A single person or a group of 2-3 people would have less people to please, allowing more innovative thinking.
I've been reading a lot lately about people who feel like there are too many individuals involved in the advertising process (agency and client side). Some feel like these numbers are preventing a lot of good work from coming out. Basically, by the time an idea is finalized and executed, it barely resembles the original idea because it has gone through so many hands. What is left is some generic piece that is struggling to satisfy everyones needs. I think that's a problem.
PsyBlog has a list of remedies to help improve your brainstorming sessions. You can also read the full article through that link.