I really like this little addition. Very good job with it.

My suggestions are as follows:
1. Like the suggestion below, give authors the ability to create a kind of dialog with the users. For example:
Insert tag, Prompt user for some parameters, throw the parameters into the appropriate predefined position. Useful for things like... [ youtube:PasteURLHere ]
2. We need to be able to delete Categories as well as adding them. If there is a way to delete them, I wasn't able to find it.

Thanks, this is a really good addition!