Ooh, I've got a new one: most metal covers of pop and "mainstream" rock songs are usually terrible. I think it's because the original choice of chords, lyrics, and melodies convey a sense of brightness and joy and that doesn't really work well when you slap super distorted guitars, fast drumming, and screamed/growled vocals underneath it. I don't think I've heard a metal cover of a pop song that I would say unquestionably improved on the original. Very few are even on par with the original.
I don't know about "most," but you make a good point about why many such songs don't work.
I've heard a couple covers of Toto's "Africa" that I think are great.
Also, last week, a guy at work was talking about Chris DeBurgh's "Don't Pay the Ferryman" and asking if I liked it. I've probably only heard it a couple of times in the last 30 years, but the song was all over MTV when I was a teenager and spent hours a day watching MTV. I said that it was a good song and suggested it might make a good metal song, so we went to YouTube and listened to a half dozen (or so) metal covers. About half were really good and the other half not so much. The half that weren't good tended to overdo the distorted guitar and had Cookie Monster vocals -- i.e., no subtlety at all.