Has to be easy to use, easy to write to in particular, and quickly searchable.
Resist all attempts to impose process or standards to wiki entries. If that fails, create a second, "draft" wiki immediately.
A few people are quite disinclined to write things down. A few others are quite inclined. Most of us are in there middle. It's important to remove barriers and make it easy. Think of it as a product that users have to want to use.