I think the word "rebellious" hits it right on the head. Music that tries to push forward and stir things up. That tries to rise above what has become ordinary and mainstream. What was once rebellious becomes the normal. It can still be good music, but no longer rebellious. A lot of the pop-punk bands sound good and are exciting to hear, but are not really changing things.
Rockin Rod Strychnine said:
So how come we use the word "punk" instead of "rebellious" when that's what is implied?