Hmmmm.... Although these guys don't really give an answer, they remind me of some other bands that fit my definition:

http://www.progrock.net/what.is.prog.html

- King Crimson (by the way, I'm going to see Tony Levin tomorrow night at the Boardwalk in Sacramento, woohoo!)

- Allan Holdsworth

- Frank Zappa

I agree with their implication that The Cure, The Smiths, and Nirvana are not progressive rock.
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Tony Fabris