Nothing gained but titles, most of which are luck based. No skill points, no special unlocks, nothing else.
No joining friend's games.
No being able to choose which campaign you want in on.
No being able to hunt in Ada's campaign.
AI players being gods.
AI infinite ammo.
AI accuracy that can shoot a single strand of hair from 4 miles.
HORRIFIC controls as most mobs.
I enjoy it but I have the sense to realize that the mode is pretty much broken beyond repair already.