I just edited some tag wikis to explain what the tag was for. I did it for tags like pcp and qma where it may not be clear what the acronyms stand for. So for example, for QMA, I explained what it stands for and linked to the Wikipedia page. (See the tag wiki for QMA.)
I think it would be nice if tag wikis contained enough information to decide what the tag is about. For some tags this is obvious, for other like qma and approximation-hardness, one could just explain that they stand for Quantum Merlin Arthur and Hardness of Approximation respectively, and link to their Wikipedia page, or their page on the complexity zoo, etc.
Is there anything else people feel that tag wikis should contain? For example, StackOverflow's tag wikis are huge. See the tag wiki for PHP for example. At this stage of the site it seems unnecessary to have such long tag wikis.
So this is my suggestion:
A tag wiki should explain what the tag is for (when the tag name doesn't explain it well enough) and perhaps contain a reference (to a website or book) to explain what the concept is. Usually Wikipedia links should suffice for this purpose.
Suggestions? Comments? Also, if your reputation is not high enough, you can request tag wiki edits in this thread, and someone who has enough rep can make the changes.