>Your absolutely right. I wouldn't want to have checkboxes and check for those you want to have. I would want checkboxes and check for those I don't want to have. I don't think the category list is going to grow into the thousands. If it does I really don't want to have to select from a list the ones I don't want because I guarantee you I will not want almost all of them.

Well, it's the same thing for other filtering options. For example, you wouldn't want to check all members you want to read messages from. So, you only select those you want to filter and they get added to the filtered list. The same applies for the Category filtering. The title says it all. "Filtering" not "Participating". :)