I think it would be a nice feature when categories could be closed, so no new blogentries could be filed under that category anymore. Closed categories should not be visible in the list of categories and return a proper message when a blogger tries to file a blogentry to a closed category.
Why? When you maintain a blog for a sportsclub with different seasons and want to categorize blogentries per season (which is a categoty and has different subcats like match and friendly) you get too many categories with subcategories after some time.
When there is a possibility to close then (so they are hidden from open cats) you have a better view selecting the categorie you do want to use!