It seems like a lot of tags that are anything other than "series-name" become the subject of debate in meta. So, what other tags are actually OK? You don't really need to list every other tag that is OK. It would probably be more helpful to describe what makes some of these other tags acceptable.
I raise this question because it seems a little odd that so many questions only have one tag (their series name).
To quote Madara Uchiha (the user, not the character):
If we wanted series only tags, we would have to call them "categories", don't you think?