My friend once told me that he met someone at a show who only listens to Death Metal, but not just that-he only listens to Death Metal that MAKES him feel dead. Depending on how you feel about that will probably indicate how you feel about this statement: I consider myself an open-minded metalhead. In my approximately 15 years in listening to metal, I've expanded my tastes to other types of music including folk, jazz, alternative country, and indie rock amongst others. I don't think this fact makes me any less of a metalhead, nor does it make me a "hipster" (note: if someone were to actually call me a hipster, I would abandon my pacifist ideals faster than you can say "That's ironic"). After all this exposure to other types of music, I'm still happy to say that I would rather listen to a mediocre metal band than a mediocre band from almost any other type of music (meaning, that metal is still my favorite type of music). However, I think any metalhead being honest with him or herself would have to acknowledge that metal, no matter what subgenre, offers only a limited palette on the emotional spectrum. If you are satisfied by said palette, then more power to you! If not, then I can provide some suggestions for metalheads looking to branch out:
Often erroneously lauded as "The Canadian Bob Dylan" for his expert storytelling, Cohen is far from Dylan in almost every conceivable way...OK fine, he's like Dylan's sad bastard fifth cousin. Leonard Cohen has written some of the darkest music in mainstream folk music, covering topics of suicide, infidelity, love, death, and religion amongst others. He most famously composed the tune "Hallelujah". While that song has been covered to death, his vast career has much more to offer than that. His lamenting, untrained voice and intimately personal lyrics is what first attracted me to him and allows his music to come through without pretense. Instead, his music evokes a feeling of very real human fragility and vulnerability.
Recommendations: "Songs of Love and Hate" and "Songs of Leonard Cohen"
Probably the most obvious choice in my list. Of all of the non-metal bands that get mentioned as influences for metal bands, Swans may be the name most often dropped, and for good reason. They are often claimed to be the progenitors to Post-Metal and Industrial Metal, while also greatly contributing to the beginnings of Gothic Metal. Swans began their career in the early 80s as part of the No-Wave New York scene. Their early sound was bleak and unforgiving. Their songs were often slow, dissonant, and metallic with Michael Gira's uncompromising baritone howl expressing perspectives of abstract violence, sexuality, and identity. Later, Swans refined their sound through a number of different musical transitions including Gothic music ("White Light from The Mouth of Infinity") and experimenting with found sounds ("Soundtracks for the Blind"). Luckily, Swans has reformed with their newest album "My Father Will Guide Me Up A Rope To The Sky" which matches the quality of the earlier material, with more orchestrated aspects.
Recommendations: I personally prefer their mid-late period, basically "White Light..." and on, highlights being "The Great Annihilator" and "Soundtracks for the Blind", but "Children of God" is also quite good. Check out their early material for more challenging aural experiences. Also, I would recommend checking out Michael Gira's other band, Angels of Light, which has a more folky sound heavily influenced by...you guessed it, Leonard Cohen.
90s Post-Hardcore and Math/Noise Rock
It was hard to pick one band out of one of my favorite periods/styles of music, but for me, bands like Polvo, Chavez, Drive Like Jehu, Jawbox, Jesus Lizard, Castor, Fugazi, Shiner, and Hum have made some of my favorite music...ever. It's incredibly unfair to lump all of these bands together because they all sound so different from one another. What most of these bands have in common is noisy, guitar-driven rock/punk music with whacky song structures and time signatures, but still retaining some sense of melody beneath the maelstrom. This balance between dissonance and melody is something I've striven for in my own music writing for years. Many of these bands would go onto influence the Noisecore of the late 90s and early 00s.
Recommendations: "Better Days Will Haunt You" by Chavez, "Yank Crime" by Drive Like Jehu, "Lula Divina" by Shiner, "Today's Active Lifestyles" by Polvo, "In on the Killtaker" by Fugazi, "Goat" by Jesus Lizard; and for those willing to endure a bit of "emo" vocal stylings, "Downward is Heavenward" by Hum and "s/t" by Castor.
For me, most progressive rock is just far too pompous, melodramatic, and even worse-cheesy. King Crimson is the only progressive rock band that I have consistently listened to and I can safely acknowledge them as one of my favorite bands without blushing. What makes them different than most progressive rock bands is, at least in their early period, they had a darker sound with minor third chord changes, flat fifths, and heavy metal-like distortion. Guitarist/songwriter Robert Fripp has always employed the most impressive musicians in King Crimson, not to mention Fripp himself who is widely considered one of the best guitarists of all time. King Crimson has gone through so many musical transitions it's hard not to find at least ONE album you like in their catalogue. I think most metalheads would prefer their early period, but I prefer their "new wave" period, with the apex of that being "Discipline".
Recommendations: "Red", Larks' Tongues in Aspic", and "Discipline"
Sleepytime Gorilla Museum
I purposefully put Sleepytime Gorilla Museum here on the list because they are actually heavily influenced by King Crimson. As a warning, I'll just say that these guys are fucking weird. As an example, one of their albums is based around a fictional political dialogue between The Futurists and The Unabomber. Not exactly your normal concept album, but interesting nonetheless. Sleepytime Gorilla Museum (SGM) combine a penchant for industrial noise, progressive rock, and avant-garde classical music into some truly whacked out compositions. They've mentioned in interviews that the way they generally construct songs is by having one person in the band make some "noise", and then the other members add their own parts. This often ends up in a polyrhythmic cacophony, but there is usually off-kilter melody lurking beneath the maelstrom of dissonance. Besides all of that, they are masters of dynamic shifts. Hearing is truly believing.
Recommendations: "Of Natural History" and "In Glorious Times"
Chelsea Wolfe has been making some waves lately. She plays a dark, brooding form of folk and dirge blues usually accompanied by a piano or a guitar. She just released a new album called "Ἀποκάλυψις" which translates to Apocalypse. I really don't that much about her, but she has a beautifully haunting style, which sometimes can be downright frightening. She has also covered one of Burzum's more kvlt numbers, "Black Spell of Destruction".
Some of Chelsea Wolfe's material reminds me of Dax Riggs in a way...er, maybe that's the other way around. For those that don't know, Dax Riggs is the former lead singer of NOLA heroes Acid Bath. After the unfortunate demise of said band, Dax has forged a solo career as a dirge/neo blues rock phenom. The subject matter of his lyrics are still quite dark, covering subjects like suicide, death, Satan, and the like. The image that his music brings to mind is the dark underworld of New Orleans, or the backwoods swamps of rural Louisiana. Dirty and dark.
Recommendations: "Say Goodnight to the World"
Killing Joke has been around for ages and is still producing good music. Not only that, but they have been highly influential on bands like Napalm Death, Nachtmystium, Behemoth, and even Metallica to name a few. They are considered to be one of the progenitors of Industrial Rock and Metal. Much of their music is often simple punk/metal riffs set with a driving rhythm section, noisy or melodic synthesizers, and of course, Jaz Coleman's recognizable snarl. Coleman's lyrics often deal with the negative side of the human experience covering themes like greed and religion. However, don't dismiss Killing Joke for their perceived simplicity or their use of synthesizers. Their cold, calculating execution is like that of a serial killer: exact and without remorse.
Recommendations: "Extremities, Dirt, and Various Repressed Emotions", "Killing Joke", and "Hosannas from the Basements of Hell"
For Metalheads into bands with electronic elements, I would recommend the more-often-than-not cold sounds of Venetian Snares or Autechre. If you're interested in some darker post-punk you have bands like Joy Division, Bauhaus, and early Cure.