Thanks for the suggestions @tiegs I have added the following tags to the categories page now.
Wildlife, Birds, Sports
I could only find two entries tagged with Motorsports so I left it out for now.
Currently, we can only aggregate one tag per category. So the tag 'elephants' can not be aggregated to 'wildlife', However, we are fixing this in our next design update (PhotoBlog 2.0).
As you can see from the screenshot from version 2.0, next update will also list categories under different sections for ease of browsing. If you can think of any improvements, please discuss them here. What is your current favorite site for discovering photography content?