Four conditions to be met for collective intelligence to flourish:

• Diversity of opinion: Each person should have private information even if it's just an eccentric interpretation of the known facts.
• Independence: People's opinions aren't determined by the opinions of those around them.
• Decentralization: People are able to specialize and draw on local knowledge.
• Aggregation: Some mechanism exists for turning private judgments into a collective decision.

See the wikipedia page for more.