I think Left 4 Dead 2 is going to be a less serious game all round. Though, that may just be the first campaign, the parish. I think each one is getting more of a separate feel this time. The parish is slightly more jokey, with zombies wearing Hazmat suits, southener music mixed in with the left 4 dead theme, a crescendo event where you activate a festival float ect.
Swamp Fever looks like a darker, more serious affair. The uncommon infected for that area are pretty scary (mudmen, they walk on all fours) it's set at night and it looks a lot more scary.
That's what I think anyway. I wouldn't mind L4D2 being less serious, it became a pretty hilarious affair anyway with all your friends online, doesn't mean it isn't scary.