I've seen quite a few tag wikis, and even tag wiki excerpts, that were copy-pasted from the first paragraph or so of the Wikipedia article, or sometimes were a dictionary definition. These violate the Wikipedia license (which requires attribution), and are almost always substandard — Wikipedia is an encyclopedia, for the general public; what we want in a tag wiki (and especially in a tag wiki excerpt) is an explanation of what the tag means for the site's community. (Related: Copy and pasting Wikipedia articles into tag-wikis).
The tag wiki edit page has a very good description of what is expected from a tag wiki, and links to Jeff's great blog post on the subject. Unfortunately, it's well-known that users don't read, and common rashes of well-meaning but misguided users submitting copy-pastes from Wikipedia as tag wikis confirm this. Even having tag wikis edits reviewed for most submitters doesn't always help; approvers need educating too.
How can we better educate tag wiki editors and reviewers (without discouraging tag wiki creation)?