I'll give it a shot. Personally the album means a lot to me because of its strong associations with a particularly formative period of my youth, but trying to put that aside and listen objectively I think there are several reasons why it's a pivotal album for R&S:
In terms of the song structures and the variety between tracks, and even within some of the tracks, it feels much less bound by genre constraints than their previous albums. Compared to e.g. the straight ambient of Amber (which I also love, don't get me wrong), Chiastic Slide really feels like they're doing their own thing.
The sound design is incredible. The sounds on the earlier albums sound like synthesizers, but the timbres on Chiastic Slide sound like something else - some mechanical, some alien, some abstract, but all in some way real. I don't know much about production but I suspect that this album is the point when they started relying less on hardware and started seriously exploring the possibilities of software.
As good as the sound design is, much of the album would still hold up without it. For example Pule with its deceptively simply melody but baffling time signature, or the jarring-in-a-good-way transition into the pentatonic second half of Cipater, or the intricate rhythms of Tewe. Those tracks could be played on traditional instruments and they'd still sound both good and original, and there isn't much electronic music about which that can be said IMO.
Of course none of that would count for much if the music didn't resonate emotionally, which for me it does massively, but if it doesn't for you then I don't suppose there's much I can say to change that. Hopefully the above gives some new perspective though.
Now I just need someone to make me understand what people like about Draft.