I believe the origin of the term is military jargon, and that it refers to the force which comes in front of all others. When borrowed to describe an artistic trend I think it means experimenting wildly with things unthinkable, unseen and unheard of, leading the next generation of artists (in case of success in becoming recognized) and our culture in general to new realms of creativity. It's a big, unexpected, shocking and hard-to-digest surprise when having an avantgardistic experirence. Maybe just experimenting and toying isn't enough to gain the avant-garde rank - it should enforce upon the listener a change in conceptions (either that, or arouse repulsion).
Avant-garde is sort of blasphemous towards current common beliefs about what music should be.
There are plenty of hardly-known, unpopular metal bands. While the music of some may be very original, it would still be misleading to crown them as avant-garde in haste, as some reviewers or fans do (imho).
Sometimes I also see people mistake something provocative for being avant-garde.
I'm not sure which of the bands I like (if at all) the definition above fits for, but usually when I see a reviewer having a hard time following some chaotic or abstract musical piece, maybe even using the term avant-garde (in a positive or negative manner), I usually make sure to take notice and check them out. The reward, sometimes, is great. (sure, there's also plenty of shit out there, as we all know (refering in a conflicting way to different bands - my 'shit' may be your favourite stew)) I'm using that common mistake to find non-avantgarde music, which is interesting and refreshing. I guess that I'd be repulsed by real avant-garde like just about everyone else.
btw, can anyone define progressive music?