In addition to their early music, I've always thought the story of Genesis as a band was kind of interesting as well. I'm not an expert on the band's history (in fact the only members I can recall off the top of my head are Peter Gabriel and Phil Collins), but I do know a little about them. I got the sense that (as in most bands I suppose), some members leaned more towards the "experimental" side, and others had more conventional sensibilites as far as songwriting, etc. I thought that Peter Gabriel was sort of the most "out there" as far as pushing the envelope conceptually, musically, and with costumes:
Wasn't "Lamb" the last album Peter Gabriel did with the band? I've been curious to hear some of the early solo albums he did after leaving Genesis, before he became a huge rock star (although I guess he was pretty big while in Genesis).
Another thing I thought was interesting about the "Lamb" album was that Brian Eno apparently contributed some of his "treatments" to it (I assume that's referring to production elements and maybe playing some instruments here and there). I guess Eno has kind of a tangential connection to prog rock in that regard, and through his association with Robert Fripp. I've listened to lots of Eno's stuff and have really liked most of it. I like how he's kind of a common thread running through glam, prog, punk, new wave, no wave, etc. That's what really interests me the most - how the different genres are connected.
Those early Genesis albums are also interesting in that they really show what a good drummer Phil Collins is. You don't get much of a sense of that from his later music. The stuff he does on "Lamb" and "Selling England" is really impressive.