With ...and then there were three..., Mike had to step up. He'd always played guitar in the band as well as bass, but this was the first album where he had to do both. Also, as sole guitarist, he clearly wanted to show that he could fill the gap. Maybe he's no Steve Hackett, but he takes some nice leads, and his sounds are very similar to Hackett's, and also to Anthony Phillips before him. I like what he does here, as well as on the later albums.
What Genesis lost when they went three-piece was the acoustic side. Those wonderful interludes with acoustic six- and 12-string guitars and Tony's piano. And it's a bit confusing as well, because that was part of the Genesis sound before Hackett joined, so there was no real reason to lose it when he left. Maybe it was a decision based more on the direction the band was going in at the time.