The idea wold be that only categories that where put up and abandoned would expire. If a user set up a unique category that only he used or mabe only a few people used that would be fine. He would only need to log on every month or two to renew it to keep it alive. A simple check box and submit. Then when enough waypoints are logged it would become permanent.
Then with some sort of rating system waypoint owners could start to get suggestions on other categories that their waypoint might be better suited for. If enough waypoints migrated out of a category and into better ones then the empty category would automatcly get pruned from the tree, no intervention required.
And as for grossly inappropriate sites, I think "site of a corpse" was mentioned earlier you report it to the staff. A good policy on that should probobly be written. It took ebay a while to compile their list of stuff you can't acution, mostly through trial and error.
I'm thinking of geocaching.com as a model. The caches are not owned by geocahing. With the exception of legal issues geocaching does not tell owners where to put them or what to put in them. They belong to the owners. The same could be said of waymarks. The owners do significant work to mark and log their waypoints and i don't think it's unreasonable for them to want to place them on the site according to their preferences.
I'm also thinking of the Waymarking staff that will have to manage whatever system goes into effect. Websites in general take alot to mannage as it is and any of the workload that they can pass on to automation and user inputs will make their lives that much less stressfull. These are worldwide sites. Just think of how many geocaches are out there and then remove the neccesity for a cache and the potential for descrete wamarks is staggering.