I'm at work so i can't listen to all but a guess would be first studio album from each? Maybe songs about depression?
No, that's not what I was referring to although some are about depression. The answer to my question which I feel confident you would have gotten had you been home and able to listen to all of them or even half of them is that they are all what I like to call "mellow metal" or "trippy metal." Most of them are only mellow in parts such as starting out mellow an then getting heavier as they go along but, for example, "Strange World" by Iron Maiden is mellow all the way through and beautiful in that "trip metal" way which I love. This, btw, is nothing you should confuse with "power ballads." A lot of them have a druggy vibe to them which is cool because if you're in the right mood, and especially if you listen with headphones, you feel like you're getting high off these songs but it's a natural high without the disgusting side effects of most illicit drugs! Natural highs rule!