My suggestion is that when someone is asking a question, certain conditions cause a message to appear advising the user that their question may be appropriate for community wiki status, and explaining briefly what this means. The conditions under which this would happen would include:

1. The title containing any of the keywords:

	* favorite
	* opinion
	* most
	* least
	* best (or possibly phrase "are the best")
	* worst
	* think
	* feel
2. Any of these tags are used:

	* fun
	* subjective
	* not-programming-related (for SO)
	* poll
	* off-topic
	* humor