Co-op shooters are this niche that is very hard to nail and L4D2 is the simplest but also arguably most effective form. What makes it have such a strong following is many different things going for it from simple to understand gameplay that promotes coordination to survive, large modding scene, and the unique versus mode where you can go up against your friends as the special infected types to impede progress. It's a shame that it never really got a sequel that builds upon it further tho I am thankful that Deep Rock Galactic and Helldivers scratch similar itches for myself in its absence.