What I like is when a singer can adjust his/her voice to suit what they sing about in such a way that you can sense the emotion back in their voice.
My Dying Bride, Sentenced, Within Temptation, Opeth, Edge of Sanity, Eluveitie, and lots more do that.
Where pop music tends to have quite 'simplistic' subjects and therefor lyrics (mostly about love or happiness) metal has always also looked at the darker side of society. Both the pain of losing someone and the harm inflicted on people by others. That is also a reason why most metal will never be mainstream as most people don't want to be reminded about life not being all that nice and simple.
Whether it was Slayer singing about Joseph Mengele (Angel of Death), Sentenced (Killing Me, Killing You) or My Dying Bride (For My Fallen Angel) singing about a loved one dying, or even a subject like incest about which many bands have sung songs, including Megadeth (Family Tree), Rammstein (Spiel Mit Mir and Tier). Dark subjects no one wants to think about, but they're part of humanity and won't go away by keeping quiet about it.
Metal has gotten a bad reputation among many religious people due to the few bands that liked shocking people and therefor sang about Satanism. But many metal bands sing about serious subjects and deserve to be recognized by more people.
To fight the darker side of humanity and the mental and physical pain people can have you need to be able to understand it, and to understand it you first need to be able to talk about it. That's one of the reasons I have high hopes for the growing metal scene in the Middle East. Whether it's the Arabic metal bands breaking local taboos with their music and lyrics, the people who make the festivals possible there (Desert Rock was allowed as metal festival in Dubai thanks to a local princess that liked 'louder' music) or bands like Orphaned Land from Israel who perform alongside Arab bands.