You're right that this isn't very useful for obscure categories with very few posts (#belegarth?) but it might prove effective for evaluating relative value among popular tags relative to other posts in the same tag. There's a list of popular tags here which might be a good starting point.

It would be a really nice metric to be able to say something like:

"This post is in the top 10 for best-written content in the #africa tag this week."

If you haven't already, I'd recommend checking out @thing-2 and @gentlebot - They're both comment evaluator bots that try to do something very similar to what you're doing with this bot.