I think a lot of those bands wouldn't be considered heavy metal by today's standards. Back when heavy metal was considered to be a sub-genre of rock music, a lot of hard rock bands were given that title as well. However, now that metal has become it's own genre and is a whole separate entity from rock, most metalheads don't consider many of those hard rock bands to be metal. Black Sabbath and Iron Maiden are for sure, but some of the others I'm not so sure about. Having trouble playing the videos so I can't really say that for all of them.
Well over time metal continued to develop away from rock, just as rock came out of blues and became it's own entity, although somewhat related. Metal now has a huge list of it's own sub-genres (which each contain sub-sub genres) to the point where it became too big to just be another sub-genre of rock.
Also originally it was just a term used to describe really heavy rock music, but the heaviness of metal just became one of many traits of the music, and it takes more than simple heaviness to describe it, because certain forms of rock music evolved to be just as extreme and heavy as metal, but the rock roots are too apparent to classify it as metal.
Metalheads frequently debate over what is considered real metal, and what isn't. For example many metalheads reject the genre "metalcore" as being true metal, because it borrows so much from hardcore rock. Not a simple matter by any means, but for the most part it's generally agreed that metal is no longer a sub-genre of rock.
I'm into a lot more of the extreme stuff, lots of thrash, black, and death metal, also lots of grind (Grindcore, Deathgrind, Cybergrind, Noisegrind) if you consider that to be metal. I try to branch out into everything though, and also enjoy some Folk Metal, Power Metal, and Stoner metal from time to time. I'm not really much into that much of the classic stuff like Iron Maiden and Judas Priest, but I love Black Sabbath.
And you've already got thrash covered with metallica, megadeth, and slayer, as well as stoner/doom metal with Electric Wizard and doom/drone with Sunn O))).
If you need explanations as to what defines each genre, I can try to help, but listening is the fastest way to tell them apart.
I've heard of those bands, but am personally not too familiar with their stuff. According to the metal archives, which is usually pretty accurate Dark Tranquility is Melodic Death Metal and Candlemass is Doom Metal.
Lacuna Coil is a pretty good metal band,Metallica,there early stuff and i wonder what there new CD in gonna be like. I heard a new song by them on the radio the other day and it didn't sound too bad,at least it didn't sound like there last CD, St. Anger the drums drove me fuckin nuts. Hell of a live act tho i'll give them that. Korn i really like there stuff,it's different and it rocks, another great live band. Venom is pretty hardcore. Hatebreed almost forgot about them guys. Check out Headbangers Ball on MTV2 on Saturday nights, i think around 10 pm. eastern it comes on. I haven't watched it in awhile maybe it's not on any more. When i did watch it i was very impressed, there was a lot of really good metal bein played on there and it wasn't all just screamin and crazyness.