I think Master Ray summed it up almost perfectly for me in the first post. Impurity was the third NMA album I bought, after it took a while for T&C to get me into the band and No Rest For the Wicked to confirm their greatness for me. With Impurity it was love at first listen and 20 years later it still sounds as fresh and wonderful as it did then. I'm sure some of it is because I'm a synth player/programmer and Impurity has the best synth work on it by far. I also really like some of the studio tricks they've used to great effect, like the fading intro to Innocence, the reverse reverb on the drums in Eleven Years and the backwards guitar in Lurhstaap.
Bury the Hatchet is probably the only song that I don't think is amazing, it's way too folky for me, but in the context of the album it really works. If I was to list my 5 favourite NMA songs (there has to be a thread for that here somewhere), three of them would come from this album - Purity, Lust For Power and Vanity (but not necessarily in that order). Those are songs for which no stereo can muster enough volume.
BTW, I should note that being from Sydney, I've never seen NMA live so there is no live bias here. They were supposed to come out here a couple of times in the last 5 or 6 years but neither tour ever eventuated. I couldn't see it working though, nobody out here knows who they are (despite my best efforts).