I´m sure there are people here with more insight in these things than me, but I have just done a bit of reading on brainstorming, groupthink and collective creativity.
As far as I can read the research, groupthink becomes problematic when members of the group are trying to "manage their presentation of self" (ie, impress). Brainstorming and other such ideas do not work well as long as the members are trying to achieve status of some kind. And most people are, most of the time. If not actively looking for status, most people are (thank God) somewhat socially self-conscious.
I guess the problem with design by committee, is that you tend to get everyones veto-es, rather than everyones ideas.
However, collecting the ideas and knowledge from a larger team is usually a good way to higher quality (though certainly not a time-saver). Apparently, the trick is to find a situation where people generate and communicate their ideas outside of a collective group context. Brainstormings work better if the group is asked to write down all their ideas for three minutes, and then hand in their contributions anonymously to an administrator that then mixes and mashes the ideas before the group again gets the ideas in return for further refinement and evaluation.
Crowdsourcing at BPL would do well to avoid a general brainstorming in the forums. Many people would feel that the threshold is too high for their ideas, and still more would go down the "lighter-than-thou" path. But getting members of BPL to share their wisdom and experiences will certainly be useful. An anonymous poll, or a personal message system, or some other mechanism to sever self-presentation from genuine creativity would be useful.
I think one of the reasons why crowdsourcing on the web sometimes is a success, is that it is a community of strangers (more or less). You get the benefit of the group, but, if done right, not the disadvantages of status anxiety.