The mother of Left 4 Dead 2. Without melee weapons, this game is all about teamwork with firearms.
While there is less kinds of special infected, less numbers of them drops ingame, making the game pace slower, which is quite good for me.
The special infected bots are also more stealthy in singleplayer campaigns than L4D2.
One more thing, there is no more official valve servers which hosts online lobbies of this, as L4D2 became the majority of Zombie slayin' in Source engine. Only recommended if your friends already got / going to get this game, so that you or your friends can host a local lobby and play together.

The mother of Left 4 Dead 2. Without melee weapons, this game is all about teamwork with firearms. While there is less kinds of special infected, less numbers of them drops ingame, making the game pace slower, which is quite good for me. The special infected bots are also more stealthy in singleplayer campaigns than L4D2. One more thing, there is no more official valve servers which hosts online lobbies of this, as L4D2 became the majority of Zombie slayin' in Source engine. Only recommended if your friends already got / going to get this game, so that you or your friends can host a local lobby and play together.