I think the prime problem would lie in deciding when a question has been 'answered'.
In your example, Bob provided an answer, so the question was 'answered' ... but then there was a follow-up question. And what about "What Music Are You Listening To" http://www.dhammawheel.com/viewtopic.php?f=12&t=11033
- 360 posts but the original question is still open - or the monstrous rebirth and vegetarianism threads?
If your suggestion could be made to work, it would only be by adding a huge amount to the mods' workload.
But how about changing the category name, from "Unanswered Posts" to "Posts with no replies"? Sure, it's clumsier, but it's clear and unambiguous.