No single player might be a turn off for some people, but I can definitely see some places in the Campaign which just wouldn't have been possible with WFC style co op. The campaign is pretty damn good in using game play for a story telling perspective. My one dissapointment is the lack of boss battles. WFC had some pretty awesome boss fights, and there were a lot of them. I just finished chapter 10 and i haven't seen anything resembling a real boss fight.
I just wish they'd focused on making escalation better in the place of co-op. If anything it feels like a step back. Admittedly i've only played one game, but not being able to choose your character, just class, is pretty sad. And its damn easy. I never beat WFC's escalation. I managed to beat FOC's escalation my first time, with a random matchmaking group.