What's interesting is that he points out that contrary to many Yellow Pages type sites, Yellowbot didn't structure their site like most directories with very broad top-level categories. Rather, their site architecture cuts straight to the chase with subcategories that people are likely to actually search for.
Andrew gives the example of people looking in a directory for "auto wrecking" as compared to people looking for "automotive." Subcategories like auto wrecking are treated as top level pages by Yellowbot. The benefit is fewer pages to crawl through for SE spiders.
What's particularly great, in my opinion, is that from a user's perspective this also works out. That traditional bit about making everything in your site accessible within three clicks? Site architecture like this makes that alot easier to achieve. What's even better is that here, SEs are really following what humans like.
I had to question Andrew on his assertion regarding local modifiers though. In my experience with my client, the Hotel de Paris, people search for "hotel Montreal" or Montreal hotel. Not "downtown Montreal hotel" or the like. See our discussion in Andrew's comments for more (or comment here ;) ).