Okay, so the art design and sound is definitely not up to par with what anime was being broadcast at the time, but it's not the art or sound that makes this suspense anime what it is. Frankly, I found the slow tense pacing of this anime very enjoyable- it's hard to find good horror anime that doesn't immediately throw tricks at you right out of the gate. A good horror story is one that uses a tense atmosphere to make you afraid even when the monster isn't on screen. And The Dark Myth is really very good at building atmosphere, albeit it does so at a crawling pace so I understand why many people find it tiresome.
As far as the mini mythology lessons throughout the film, I found them to be very interesting and makes the film a lot easier to enjoy. I think I would have been terribly confused without them. Unfortunately, I cannot attest to the accuracy of The Dark Myth's telling of these myths, but from what I understood of it, they seemed clear and believable. There is far more talking than there is action, so I see why many anime fans would be frustrated with that as well. It seems to me that The Dark Myth is definitely geared as an adult mystery and not really something for teens.
I think that this thriller anime definitely needs to be given a chance, and fans of horror anime who are looking for suspense should watch through it at least once.
