There are stirrings at Slate and elsewhere about the true worth of progressive rock, that hyperventilated amalgam of trick pony riffs that made radio something you dreaded turning on. I don't buy it, for the most part. The fascination with progressive rock grew out of the long improvisations pioneered by the essentially blues based bands like The Butterfield Blues Band, Jimi Hendrix Experience and Cream, as the going conceit of the time was that rock and roll had become something smarter like "rock" and could now rival jazz as a young musician's medium for instrumental chops.
The transition to classical borrowings, occasional jazz motifs and jacked up time signatures and tick-tock chord changes made for its own kind of monotony. Jazz, whatever its form or origin, was premised on the idea that , as a form, it was in a state of constant transformation--the musicians we still listen to rarely played the signature tunes the same way twice. Progressive rock, generally proud and defensive about the form's gerrymandered fussiness--this was the best place to learn the distinctions between the words "complicated" and "complex"--became insulated ever so much faster than jazz did.
A one idea concept, with rare exceptions --Zappa, King Crimson, Pink Floyd--progressive rock could only become fussier, crankier, more incestuous. It actually became something resembling "rock" not at all, in any sense. It was the arena of sterile perfection and was truly unlistenable to a young listener having no desire (or need) to stare at the sky and ponder stoned philosophies. Punk rock was the shock rock and roll needed; stupid, obnoxious, repetitive, angry, the rude style pretty much revealed what a conceptual crock of mung progressive rock turned into. It was time to flush things away and allow progressive rock to become something actually useful, such as fertilizer.