To be honest, though I like much of the game, I feel the game itself too predictable. I think it suffers from having the levels being too small, and having specific trigger areas for most of the zombie hordes which can be exploited.
I miss the unpredictability of Left 4 Dead with its AI director which would adapt to what the players were doing, lull you into a false sense of security and then suddenly bombard you with zombies.
If the levels were as big as Left 4 Dead then there would be more opportunities to spawn zombie hordes when the players aren't expecting them.
Also the special zombies aren't particually interesting - just bullet sponges really. The Left 4 Dead zombies had special abilities designed to split up the player group, or to snare lone players, forcing the team to go rescue them. (I know there's the security guard one who can grab the player, but it's not a sneaky spawn like the smoker was in L4D.)
If course like L4D there are special zones where you know you have to trigger the zombies by performing set actions, but these aren't particually amazing areas or interesting tasks, unlike the final levels of Left 4 Dead which have loads of different options for tactics and different trigger mechanisms.
Another issue is that constant snow and ice is just visually uninteresting, and makes all the levels look samey.
Don't get me wrong, I like and enjoy the game, I just wish it were better.