The comparison to left4dead isn't really applicable. The similarities amount to a group of humans vs monster and that's about it.
There's an interesting leveling system which allows you to unlock more characters and monsters as well as abilities.
But yeah it's certainly not genre defining or anything like that. It's just a simple, good, fun co-op game. Me and my friends only bought it because we were tired of Payday, Dota ect and wanted something to tie us over until Killing Floor 2. I've been pleasantly surprised though at how intense it is.
Man this Polygon review is a load of shite:
'The problem here isn't just the time required to unlock each new character but also the process, which forces a set amount of usage of each character's skill, regardless of how useful they are at any given moment. The only practical way to really unlock everything is to play solo matches in specific modes to grind out masteries.'
I've already unlocked the second character for medic and the tier 1 skills after a grand total of about 10 matches. Amounts to only a few hours play. And the second tier of the 1st characters skills, whilst more specific to unlock, aren't anywhere near as bad as this review makes out. It's really not that fing hard and definitely doesn't need solo play.