Kind of running out of steam
I will start by saying that I have loved this series. I DVR it every week and get excited to watch the episode. I haven't yet watched the power metal episode, but it seems like he is running out of sub-genres and really stretching it these last few episodes. I would say that the Shock Rock episode is a good example of this. I don't really consider that a sub-genre at all. Pretty much every single heavy metal band that burst onto the scene with something new and innovative was shocking to parents. Every hairband was shocking until the mainstream became over saturated. Every thrash band would shock parents if they actually saw or heard them.
While I haven't watched the power metal episode, I feel like that isn't really a sub-genre either. Are there any thrash or death metal bands that are not powerful? Were not most of the NWOBHM bands powerful? It seems like an excuse to show bands that weren't already covered. Since AC/DC isn't American or British, we'll lump them in the "power metal" category. Since Dio wasn't part of NWOBHM and wan't a hair band, but was from the '80's, we'll call them a "power metal" band. Is Dio any more powerful than the NWOBHM bands?
I feel like they could have had one more episode after the Nu-Metal that just brought everything together to a conclusion and highlighted some of the current heavy metal bands. I didn't even mind the grunge episode because I really think that some of the grunge bands could be considered "heavy metal" whether they liked it or not. I just think that maybe there was a contractual obligation to film 11 episodes, and they are stretching to get there.