Well, I consider a thread "high quality" if it results in people coming to the site and/or registering based on a search engine search result.
For example, our technical article threads on CBRXX.com bring in a lot:
http://www.cbrxx.com/honda-cbr-xx-technical-articles/
My "Big-Board" site that I recently sold got a lot of hits from the articles also:
http://www.fireblades.org/forums/articles/