I find I'm rejecting a lot of suggested edits of tag wiki excerpts because they lack usage guidance.
I think that part of the problem is that the explanation of what should be in the excerpt is off to the right:
What are Tag Wikis?
The usage guidance, or tag wiki excerpt, is a short blurb that describes when and why a tag should be used on this site specifically.
But that's visually separated from the user's attention; right by the edit field there's just
Usage guidance (excerpt)
Perhaps this label could be extended with a reminder (cribbed from the reject reason):
Usage guidance (excerpt) - describe why and when this tag should be used