Hi John,
I understand what your getting at with your average outbound PR metric, but I think it is a bit misleading. It's quite possible for a directory to list quality sites that are not well promoted and have low PR (especially if the directory has editors that are adding sites on their own and not relying solely on submissions).
When I look at a directory, it is fairly easy to tell if there is any editorial integrity (assuming that it is not brand spanking new and has some listings in it already). Some things I look for to gauge editorial integrity:
- Category structure - Is it well thought out or just thrown together? Is it logical and easy to browse (yes, I'm one of the few people that prefer to browse a directory than use the search feature)? Are there 1,000 listings in a category that really should be sub-divided into several sub-categories?
- What's already listed - Are the sites that are listed in the category that I think my site belongs in sites that I feel are related topically to my site? For a directory page to be considered an authority/hub, it's got to be tightly focused and comprehensive for the topic. This is why, IMO, many niche (/hobby/one page) directories perform better in SERPs than general web directories with similar categories.
- What the existing listings look like - Are the titles all the same keyword laden phrases that make it hard to distinguish one listing from another? Are the descriptions consistent in form?
- AdSense or other PPC ads mixed in with listings - Do I really need to explain this one?
Also, ! Ask !, I like what you are doing with ensuring unique descriptions for your directory. Something like that shows me (as a webmaster) that you care about your directory (and are likely to be promoting it and trying to build it into a useful resource).