What an utterly bizarre comment. Did you not know that the lead singer of one of the most successful heavy metal bands in the world, Iron Maiden, is a trained opera singer? As for the riffing, it's really no different to the kind of virtuoso playing you'll find by such guitar luminaries as John McLaughlin, Steve Howe or even Steve Hackett (listen to the riff he played on Hogweed, itself an inspiration for Iron Maiden's very own Phantom Of The Opera); it's just amplified louder.
"Singing like the devil" indeed. Did nobody ever tell you that the devil has the best tunes?
While I can appreciate the talent most metal guitarists have, I have to agree with Noni and say that their repetitive riffs can sound a bit monotonous. As for metal vocalists who are trained opera singers, like Bruce Dickinson of Iron Maiden and James La Brie of Dream Theater, I personally cannot stand that style of singing. Screaming your lyrics is not a talent. I do enjoy some of the classic metal bands though, like Uriah Heep, Scorpions, Triumph.
Now, if you move from the popular rock and roll metal genre and into the less popular progressive metal genre, there are numerous bands that I enjoy; ie, Riverside, Ayreon, Haken, etc, due to their ability to make the listener think, with lyrics that rise above those of the popular rock and roll metal bands who tend to promote a lower consciousness by singing of depression, devil worship, apathy, etc. I think the progressive music genre, in general, displays a much more positive attitude in their music than does heavy metal.
As Noni was saying, one person may not always like what another person listens to. I certainly don't.