I figured out part of it!
It looks like there are a limited number of authorized 'tags' at this time.
I went to the bottom of a page of a thread, and clicked on Tag Cloud. That took me to a page of the user defined topic lists. It also showed a tag cloud with various words in it. I still don't know how a word gets into the authorized tag cloud if it is not the title of a user defined list.
So, I went back to the November Doll Quilt Swap thread, and was able to tag it Miniature Quilting, since that is an authorized tag. And it worked!