Kyle, i find prog rock to be hard to nail down as a category because except for a few bands like yes and king crimson, many of the bands accused of this tend to be more complex then that one label purports...
I tend to use it as an adjective that means the music jumped the shark in terms of ; musical virtuosity , far reaching lyrical oddessys; and keyboards being usually a  (if not the ) primary musical statement....in short all these elements pushed to a point of pretension that it overwelms the music..... That said I love early, genesis, , Yes, King Crimson, Moody Blues, Pink Floyd.....all bands that at one time or another get roped into that category, and yes have aspects of this, but alsso have great melodic songwritting.....
It was an contrast to the 90's use of irony as the overwelming musical statement....until Arcade fire brought back that it was ok to be sincere in your music.....   but.... you know....... Arcade fire are a unimportant and overrated band...
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