I grew up with this music, so of course I think a lot of it is great. Of course there was also a lot of crap made then, all you have to do is look at a top 40 list from that era.
But, I think one of the reasons they were great is the music was new. There simply had never been long guitar jams until Cream and the Allman's did it. Yeah, they copied from old black blues players, however they took the music much farther. The chord progressions (riffs) on the guitar hadn't really been used together like they were during this era. I mean who had recorded a guitar part like in "You really got me" by the Kinks, before the Kinks?
Art rock, jazz rock, country rock, this was all new and created during this era. Everything else later is imitation. Of course Lennon said everything after Elvis and Chuck Berry was imitation.