Old Guard and Avant Guard

[hagsrus.livejournal.com]

I understand somebody commented that The Gift of the Moggy and my other writing is "old guard" -- which I take to be descriptive, not negative -- but it's not something I'd ever thought about. "Old guard" seems generally to be used to describe attitudes toward the fandom rather than its content, but it's quite likely I've missed something.

I assume the opposite is avant guard.

I'd be very interested to know what might put a story into one category or the other.