In all honesty, this is just as amazing and essential as most people say, but for decidedly different reasons than they generally state. In my experience, there's really no other black metal band that has been able to capture the same mood and style portrayed on this record, and on top of this, that very mood itself was mastered by Mayhem in these eight tracks, so perhaps it's for the best that others aren't even trying. It's amazing how even at this stage in their career, Mayhem was busy smashing the conventions of black metal and displaying the true potential of the genre as an artistic force that nearly every other artist pales in comparison to. Mayhem are not at all the godfathers of black metal post-'Deathcrush', but instead are the wild, untamed masters at the edge of the genre who deserve more reverence than they truly get.
I don't feel like going on for ten pages about how it changed black metal (since I don't think it really did much at all) or how awesome it is for whatever reason of the moment, so let's just look at a few specific factors that make this such a great and mandatory release:
-Attila's vocal performance is bar none one of the best in black metal. If you dislike Attila's vocals, you have no business judging music at all. They perfectly fit the atmosphere and music that Mayhem created at this time. They're daring, unique, and flawlessly executed, and all those who feel that this album would be better with Dead involved seriously need to think about why they would prefer generic black metal vocals to one of the most stirring black metal performances of all time. Both strange and rich in tone and daringly operatic and melodramatic, Attila is really the critical force on this album that holds it all together. With any other singer, this wouldn't be nearly as great, because the nasal and harsh yet strangely unaggressive intonations of Attila form a magnificent contrast with the unique and utterly non-stereotypical black metal the band plays. His clean, definitively majestic wailings in the title track alone are enough to prove his quality as a vocalist and artistic force. While in many black metal bands the vocals are merely a backdrop of convention used to add rhythm, on this release they're absolutely critical to the success of the album as a whole.
-The riffcraft is just flawless and has yet to be cloned in any significant way by another artist. Euronymous was making great, thrashy riffs and gorgeous tremolo-picked melodies as well as some fantastic lead guitar, and everything he does is just perfect on this album. There are no filler riffs. Each one is as incisive and amazingly catchy as the last, and most are far more complex than the majority of black metal from over a decade later. There's nothing to say about them; you listen, you understand.
-The production is about as good as it could possibly get, with naturally produced reverb and probably one of the best drum production jobs in all of black metal. It captures the atmosphere of the music perfectly, with each instrument very clear and present, along with an awesome guitar tone that's ferocious and cold yet very melodic and tonal as well.
Really, it all comes down to the atmosphere. While other black metal bands these days are all obsessed with war and Satan and nocturnal grimness and what have you, Mayhem went in a decidedly more gothic direction than that. There's still tracks about Satan and winter and stuff but they seem expressed in a somewhat more subtle, occult fashion. It reminds me of the first Gorgoroth album on some level, with a definitely different feel from other black metal from the same time period (and from the future), which seemed concentrated on conjuring a mood in which aggression was only a part, not the whole damn thing. It's just so obvious that much more time and effort was put into crafting these songs than the vast majority of black metal; I guess that's tragic on one level, but I guess you can't expect much more.
Anyway, 'De Mysteriis Dom Sathanas' is just as good as everyone says. You should buy it if you don't currently own it. Fun fact: the opening two songs which everyone already knows by heart are easily the weakest tracks on the disc. When 'Cursed In Eternity' kicks in, you'll know where the good stuff is. 'Buried By Time And Dust', the title track... well, whatever, just go and get it.