Excellent. We are kindred spirits in this thread (so far!).
One of my pet peeves about directories are "type" categories (forum, blog, directory) that are not topically focused. For example:
Health > Chats and Forums
While this kind of categoy is appropriate for some general topic health forums, directories with these kinds of categories often don't include the "type" categories under the more topically focused subcategories (like each individual health condition). These types of categories are not helpful to visitors looking for forums on specific subjects, they aren't helpful to webmasters who want to link to useful references on specific subjects and they aren't helpful to search engines looking for hubs of topical associations.
*If* you are going to include "type" categories, they should be topically focused IMO!
Symbolic categories are very much underused IMO. How many of you get upset when people don't submit listings to the correct category? Symbolic categories can eliminate duplicate and redundant categories and guide visitors to the correct category.
For example:
Business > Software
Computer > Software
Shopping > Software
This is confusing! Better to pick one of them as the main category and use symbolic categories for the others to redirect to the main category - like so:
Business > @Software (links to Shopping > Software > Business)
Computer > @Software (links to Shopping > Software)
Shopping > Software
This is also good for improving the internal link structure of the directory.
If you want to see a good, live example of symbolic categories, spend some time browsing around dmoz.org. They have a very well defined category structure with symbolic categories (have an @ symbol at the end of the sub-cat names).
For example:
http://www.dmoz.org/Business/
lists Software@ as a sub-cat which,if clicked, takes you to:
http://www.dmoz.org/Computers/Software/Business/