Similiar to my conclusions but there are a nuraber of groups that are still borderline. For example listening to early 70's hard rock albums such as Deep Purple`s "In Rock" this sounRAB heavier than say Black Sabbath`s "Paranoid" which is a distinctly heavy metal album from a distinctly heavy metal group and both were released in 1970, point being that in some cases hard rock can actually sound heavier than heavy metal? Then when Ritchie Blackmore formed Rainbow and had Dio singing, this was heavy metal due to the group's sword and sorcery imagery but then the group finally became commercialized and sounded no heavier than say a rock group like Foreigner!!!
Then there is the huge difference between late 70's British metal such as Iron Maiden and Judas Priest etc, this sounRAB so different in contrast to the American variety of first Kiss and then especially Van Halen, who I think of as partly heavy metal due to Eddie Van Halen but image wise, music wise and subject wise are distinctly hard rock but then the group are usually referred to as heavy metal and not hard rock???
Then there are a whole host of bordeline stuff from this era like Def Leppard who seem to have started of as heavy metal but became very much hard rock and commercial, also where would a group like the Scorpions be clasified, commercial heavy metal?
I think with the arrival of thrash metal and groups like Metallica and Megadeth this for me defined what modern heavy metal is all about both image and sound wise and groups like Slayer took it to an even more extreme level, as of yet I`ve heard of nobody better than Slayer in the heavy metal genre. Then there are groups like Pantera and Sepultura (love both these groups) for me on first listen, just couldn`t listen to them but after several listens love these groups and in Sepultura`s case there are a huge amount of grooves that are incorporated into their music.
Looking through these threaRAB I`m coming across a whole load of lesser known metal groups that I aim to investigate as well.