Posts enter community wiki mode when one of the following happens:
- The body of the post has been edited† by at least five (5) different users.
I propose to remove this feature entirely, it does not serve any useful purpose in my opinion.
The decision to make a post community wiki, and to explicitly invite contributions from other users should be left to the original author. While SE sites have a wiki-like aspect, drastic edits to other user's posts are frowned upon in most situations.
Most edits fix small problems in posts: grammar, typos and smaller factual errors. Large-scale additions or rewriting of non-CW answers is not a good idea in most cases, posting a new answer with that content is often the preferred alternative.
The only thing this feature achieves is to take away the credit and possible reputation gain from the original poster if too many people edit their post. I can't actually think of any cases where this feature provides any benefit, it should simply be removed.