Duke, the story, is a sad story. A story of loss and alienation. Add to that a couple of depressing songs from Phil, one from Mike and another from Tony, and it's a pretty downbeat album altogether. Even "Man of Our Times" and "Cul-De-Sac" are downers, aren't they?
Incredibly, I've never actually thought about that before. It's one of my favorite Genesis albums if not the favorite, and I guess I've always thought of it as upbeat because of the fanfare in C that opens the album, and its reprise at the end.