I'm struggling with the term 'progressive' when it comes to music. In this case, 'progressive metal'. Guess i just notice that so many bands that are considered progressive rock or metal have many things in common with their music, which makes it more of a style/genre, as opposed to just being progressive with their music. But i guess that the point of why the genre 'progressive metal/rock' was created- it's a style. Mars Volta, Dream Theater, Symphony X, Rush, etc. If you listen to their music it's very obvious what it all has in common. But i think the proper title for this style/genre is 'Prog', a term that is used often.
Pink Floyd is progressive, not prog. Metallica is very progressive actually, they try many new things all the time and with each release evolve. St. Anger was very progressive FOR METALLICA. Tool i like to think is progressive, they evolve their music a lot and have grown and are very very original; I don't consider Tool 'prog'.
Anyyways i realize this type of things has probably been discussed like crazy already. I'm just brainstorming.