Post-rock has a few definitions. Most people consider the sound definition of 'instrumental songs played by a rock band that slowly build up to a crescendo and then do it all over again'. This is a valid definition, after all, from a point and onwards that's what most post rock bands sounded like. I see your Mono and trade you my Explosions in the Sky, zero sum.
However earlier on, post-rock seemed to be an attempt to subvert and/or invert the tropes of the rock genre while using its own means. Hence, since rock and roll is generally considered a physical sort of music, we'll write songs that are terse and lacking in groove. If rock and roll depends on charismatic frontmen, we'll have none of that. If rock and roll depends on verse/chorus/verse structure, we'll just write long suites.
Then we have post-metal. It certainly also subverts similar rock cliches, but in a very curious way, not too many metal tropes. Sure, rock music has tame structures, but there's very adventurous metal music (structurally) and there's been such for a while now. Whereas the adventurous rock music (the more outre types of progressive rock) is comparatively outside its mainstream, in metal terms, Metallica and Iron Maiden with their 12 minute songs are right there in the middle. So, for a post-metal band to write a long suite doesn't subvert anything in metal terms. Or say, repetition. The original second wave black metal bands (completely and totally the opposite of 'post-metal' anything) were droning on the same riff ten years before Pelican.
What are the metal music tropes post-metal could really subvert? There's guitar solos, and indeed most post-metal bands do not have too many of those. But still, guitar solos are not the pure domain of metal music. There's double-bass drumming and constant palm-muting, really. That's pretty much it. Harsh vocals are also very punk-related. Curiously, post-metal bands do not shy away from double-bass, muted chords or people yelling at all. There's melodic (and usually high pitched) singing, and indeed post-metal bands do not attempt anything like that most of the time. If there's a pattern in post-metal choices, it's hard to detect.
But I do submit that there is a pattern, and it's a very schizophrenic one.
1. Post-metal bands understand - and communicate - with various types of rock music much more than they do with metal music.
This has to do a lot with how metal music was coming out of an aesthetic depression so to speak circa the end of the '90s so nobody knew exactly what metal music was (outside of black metal which was startlingly clear to identify). Post-metal bands then are in a dialogue instead with the same old common rock and roll tropes, and some of the time punk rock tropes in particular. A heavy metal guise is a tool utilized, it's not the point. Consider arty punks that perhaps like a couple of metal bands -let's say Celtic Frost and Metallica- and they're subverting rock and punk tropes by playing with a few metal sonic identifiers like double bass and palm muting. In effect this means post-metal most of the time is post-punk or post-hardcore in most of all, post-indie. This is probably infuriating to real punks that consider their post-punk to be a historically different thing, but if they could be open-minded for a minute here...
2. Post-metal bands are not subverting metal tropes as much as they're incapable of achieving them in the first place.
To play a complicated and considered guitar solo takes a certain degree of capacity that the metal musician has to hone for a few decades. Heck, to have two guitarists playing interesting harmonies without it all coming out like mud takes a certain precision too. So, no soloist? No solos. No guitar bros for life? Single guitar. No audible bass tone? No bass. A disturbing aura of an autopsy permiates. Most post-metal bands, for all their rhythmic or compositional graces, do not seem to employ any talented lead players. There is no trope to subvert when one is incapable of scrutinizing the trope first-hand. Likewise with a talented lead singer that's willing to span a range. Not too many of those around, and those that do exist would be more interested in actually playing to the trope than to subvert it (for good or worse).
This is revealing for post-metal, especially why long-time metal music listeners feel ill at ease with it. Post-metal is the outsiders looking inside metal music for incidental reasons, it's not forward thinking metal music made by passionate metalheads trying to come up with something new by testing limits.
A case could be made that whatever actual post-metal experimentation occurred, it happened in the early '90s with the short rise of progressive metal instead.
Isn't it startling how little time post-metal listeners have for progressive metal and its meandering solos, overdramatic singers and complicated song structures? It's because these post-metal listeners aren't actually interested in metal music, they're interested in the warped reflection of other rock types on the metal mirror. This also explains why although most real metal listeners were captivated for a spell by the types of Pelican or Isis, ten years down the line do not listen to that music much if at all. It's not just because they're getting older and more conservative, it's because it's exhausting to confuse the outsider perspective for the inner one. It leads to alienation from one's own core ethos.