Here are some things that are important:
Playfulness -- people need to be able to fool around. Being serious at all times creates a stifling atmosphere where people hold back and prefer to share ideas through other channels. This in turn prevents reciprocal inspiration. Take care that brain-storming is used as a tool, not as a permanent state, however. After collecting ideas, write, consolidate, rework. Use it to produce something. If you do not, playfulness does not forward the cause.
Openness -- people tend to look for the truth and to form strong opinions, which tends to stifle creativity. The formation of hierarchies and the acceptance of authorities within groups works similarly. So set some signal and forget about all these barriers when you are looking for new ideas.
Diversity -- similar people will create similar ideas. It's good to have different people from different cultures, different countries, knowing different languages, having different values in your community. They are like vectors looking in different directions and pointing into a multidimensional space of creativity. Try to allow and integrate diversity.
Friendship -- knowing other people in the community is important (it helps to cope with openness and diversity). This involves knowledge about private lives from OnlineDiaries, pictures. Being friends in real-life also helps. Face-to-face meetings, drinking, eating, and socializing are very important factors. The resulting lowering of inhibitions can facilitate brainstorming and help people to AssumeGoodFaith.
Feedback -- getting feedback fast is important, because then you can react to something, your partner can react to your reaction -- the talk-think-review cycle is shorter. This is also a reason why InternetRelayChat is better than a wiki, and a wiki is better than a static website. It reflects its users faster.
Ambiguity -- encourage people to assume that they understand each other, and not to worry about verifying mutual understanding until the creative process has come to a climax. This can be especially powerful if the people involved are using subtle or ambiguous language (or both!). If all of the other conditions are right, the resulting ambiguity (and even misunderstandings!) can foster 'migration of ideas' into unforeseen but potentially valid new contexts.
The technical infrastructure can encourage or hinder these aspects. See AppropriateMedia.
On a MailingList, people have have more time to deliberate, therefore their postings will be more serious in tone and hinder playfulness. On the contrary, FlameWar""s often occur unless people are more experienced with the medium. In order to counter this, face-to-face meetings to strengthen friendship might be in order.
On InternetRelayChat, people are used to nick names, which increases playfulness. The immediate and transitory character of IRC and InstantMessaging also creates the illusion that no AuditTrail exists -- which increases playfulness yet again. See ImportanceOfIdentityInOnlineCommunities for more about nick names and real names.