My three major complaints that kill this game are as follows (scroll to bottom for tl;dr):
1. Zero substance. No real effort was put into Back 4 Blood. It's just a short cinematic followed by players going from point A to point B while killing zombies followed by another short cinematic. The story is so nonexistent, I don't even know why I have to do all that. Also, for a horror/survival co-op shooter, it's never scary, and you always have enough resources so there's no actual feelings of dread.
2. Players are pushed to clear levels as fast as possible, which, if they want the extra rewards for doing so, makes exploration impossible. The reason for this? Probably because each level has such little substance to it.
3. Finding three other co-op partners is difficult. Hell, getting one other partner is tough, and playing it with bots just sucks.
tl;dr: Lack of substance, levels and gameplay that just aren't fun, and being forced to play with bots are the only things that make Back 4 Blood scary.