Although L4D campaign system can be done single player or co-op seamlessly, pretty sure that's the connection.
People have to start thinking less about "it's like with like and like" and more about how this game is bringing amazing NEW stuff to a probably perfected shooter gameplay.
The problem is there aren't many new things you can put into a game anymore. What makes a game innovative these days is having a unique way of combining features. Parkour has been done in Mirror's Edge and Assassins Creed; involved storylines are common, but not in multiplayer; Co-op has been done, but 8 person co-op is rare; character, skill, and weapon customization are seen often in RPG based games, but not in PvP FPS games; exaggerated colorful artstyles are rare in FPS.
Sure, you could say that Its a lovechild of Mirror's Edge, Shadowrun, Cod Blops, TF2, W:ET, L4D, BF2, or any other combination of games, but the list starts getting so long that its easier to just say that its unique and innovative.