thinking some more about Tortoise and where they went astray (the TNT album was unkindly compared to Spyro Gyra by one reviewer, I recall)... well, they certainly backed away from the studio-laboratory mixology that made "Djed" and "Gamara" so thrilling (they still sound surprisingly good, surprisingly pregnant with promise) and they played up the real-time playing instrumentalist noodle aspect (perhaps strategically tacking away from the post-rock ideology being plooped on top of them, who knows)..
but perhaps the warning sign was as early as the debut Tortoise album and the track "Ry Cooder" -- not the sound of the track, but its title
See, it struck me recently that Cooder was the prototype of a certain kind of eclectic, over-educated musicianly sensibility that's fatally prone to the lure of the Archive. (Costello is perhaps the New Wave counterpart, but then with him and the similar pastiche-leaning Nick Lowe, it's a thin line between Old Wave and New Wave -- and that line is called pub rock).
Session musicians, as a class, never really innovate because they know the correct way to do things. They need to be put to the service of someone with a vision, often a non-musician (see Eno and what he got out of all those Fairport and Brand X et al players on his solo albums). Musos need to be jolted off the tracks that have been laid down for them by all that skill and craft and learning and technique they've accrued through their wood-shedding and hireling labors. Knowing the correct way to do something has an in-built bias towards non-innovation, because the right way to do things is always the time-honored way of doing things. And being eclectic in itself--joining the dots between all these different idioms, these different crafts each with their own correct, traditional way of doing things--is rarely enough to really push you into the zone of the unknown, the New. (Band who can't play that well, who are widely-listened but not capable of successfully emulating those other idioms, let alone seamlessly integrate them, are possibly far more likely to achieve this, through their very failure).
But it's not just the virtuosity syndrome, that null zone where diversity and versatility meet: tex-mex meets blues meets soul meets folk meets hawaian meets gospel meets cajun meets zzzzzzzzzzzzzzzzzzzzzzzzz. The Cooder albums are crammed with covers and reinterpretations of older songs. (Are there any originals in there?) Sometimes the presentation and framing gets close to outright retro.
Then there are the wholesale excursions into older styles/eras (c.f. Costello, Almost Blue)
Tortoise never succumbed to this kind of thing, but you could almost see their music as a battleground across which archival-tendencies and now-aware tendencies are battling it out -- the burgeoning reissue/back-catalogue-explosion/crate-digging side of the Nineties competing with a different kind of record-geeky music-fiending of the type that leads you to import-dense dance specialists, dancehall basements with 7-inch singles on the walls, etc
the sonic composition of the band's palette in fact resembles the ecumenical stocking policy of your typical cool music store in America, like Mondo Kim's or Other Music in New York...
so to bring it around to where i started:
the UK version of post-rock was like postpunk, in so far as it was largely (if not completely) in dialogue with "other musics" of the present -- hip hop, techno, jungle, Warp-y stuff... with flavours of kraut and dub and ambient and concrete in there too, but overall a definite slant in orientation towards the NOW
(c.f. postpunk's dialogue with current black music: reggae, funk, disco -- and with the most technologically advanced European pop -- Moroder, Kraftwerk... and even with those few of its influences that weren't brand spanking new, they were relatively recent and relatively untapped: Can, Fela, Bowie's Berlin trilogy)
the US version of post-rock was a lot less like postpunk, because it was much more attuned to the archive (including things like Cluster, Neu! etc, i.e. no longer relatively recent, nor indeed relatively untapped) .... and it was much less in dialogue with its own local current black music vanguard (hip hop) let alone with jungle, techno etc
the difference here is a relative degree of present-mindedness