These are what we call the 'YMMV items.' Things that some people find in this work. We call them 'your mileage might vary' because not everyone sees these things in the same way. This starts discussions in the trope lists, a thing we don't want. Please use the discussion page if you'd like to discuss any of these items.
Awesome Music: De Mysteriis Dom Sathanas is the most frequently cited contender, obviously. Where you should go after that depends on your musical inclinations. If you want raw, old-school black metal, your best bets are Deathcrush, Wolf's Lair Abyss, and Ordo ad Chao. If you want more experimental, avant-garde material, Grand Declaration of War is probably what you're looking for (although you should check out Wolf's Lair Abyss in this case as well... and then check out Chimera).
Epic Riff - A lot of the time, most notably on De Mysteriis Dom Sathanas.
His successor, Blasphemer, was if anything even more skilled on guitar. Pretty much all the recordings he made with Mayhem (specifically, Wolf's Lair Abyss, Grand Declaration of War, Chimera, and Ordo ad Chao) have their share of Epic Riffs as well.
Evil Is Cool - Arguably part of the reason some fans like them.
Epileptic Trees: Was Euronymous really a satanist? Was he really the evil, sociopath that he made himself out to be? It's debated as to whether his twisted acts (such as using the photo of Per 'Dead' Ohlin's dead body after his suicide as a cover for a bootleg album) were because he was like that, or was it just part to build an image. Varg of Burzum keeps stating this, but due to Varg being Varg, it's debated.
Face of the Band - Dead and Euronymous, due to their untimely demises giving them a sort of evil mystique.
Growing the Beard - Varg Vikernes claims that this happened when Hellhammer and Dead joined the band: "Until Hellhammer and Dead joined the band in 1989, Mayhem was a shitty, ‘fun’, hobby-band. If you read the lyrics on their Deathcrush album you’ll know what I mean". Others may have their own opinions on when this happened, though in any case DMDS is almost universally considered to be better than Deathcrush.
Love It or Hate It - Amongst fans of the black metal genre as a whole, everything Mayhem recorded since De Mysteriis Dom Sathanas is like this. ...Sathanas is almost universally regarded as the pinnacle of the genre (or at least, of the genre at that point in time), but the polarizing reactions to their more recent work is absolute, and best illustrated by Ordo Ad Chao (a solid attempt to get back to their darker, lo-fi roots, or a particularly lazy rehearsal tape with full production) and most of all, the aptly named Grand Declaration of War (genuinely avant-garde prog-black metal which showed a willingness experiment with unusual time signatures, guitar techniques and electronica or Jesus Christ that chap spent the whole album just talking!).
Magnum Opus - Fans consider De Mysteriis Dom Sathanas to be this; it's often regarded as the greatest black metal album of all time.
Deathcrush, while an EP, is also a frequent contender.
The Woobie: Yes. Even for this band.. Arguably Dead. He suffered sleep apnea as a child and internal bleeding, which resulted with his obsession with death and the occult. Old band members that knew him (Fellow band member Hellhammer and Faust of Emperor) stated he didn't seem happy with life and Euronymous just kept adding and add more psychological problems and causing fights with him (at one point, Dead had to sleep freaking outside because Euronymous was tormenting him), until he finally killed himself. And what did Euronymous do? TAKE FRIGGEN PICTURES OF HIS DEAD BODY.
There is a bittersweet thing about Dead is that despite his death, fans and old members still consider him a fun guy, just a bit into the dark humor.