⣿⣿⣿⣿⣿⣿⣿⣿⣿⣿⣿⣿⣿⣿⡿⠿⠛⠉⠀⠙⢿⣿⣿⣿⣿⣄⠀⠀⠀⠀
⣿⣿⣿⣿⣿⣿⣿⡿⠿⠛⠛⠉⠉⠀⠀⠀⠀⠀⠀⠀⠀⠻⣿⣿⣿⣿⣷⡄⠀⠀
⣿⣿⣿⣿⠋⠁⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠘⣿⣿⣿⣿⣷⡀⠀
⣿⣿⣿⣿⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⢹⣿⣿⣿⣿⣧⠀
⣿⣿⣿⣿⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⣿⣿⣿⣿⣿⡄
⣿⣿⣿⣿⡀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⢀⣿⣿⣿⣿⣿⣷
⣿⣿⣿⣿⡇⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⣼⣿⣿⣿⣿⣿⣿
⣿⣿⣿⣿⣿⣿⣿⣿⣿⣿⣿⣶⣤⣀⠀⠀⠀⠀⠀⠀⠀⢀⣴⣿⣿⣿⣿⣿⣿⣿
⣿⣿⣿⣿⣿⣿⣿⣿⣿⣿⣿⣿⣿⣿⣷⡀⠀⠀⠀⢀⣴⣿⣿⣿⣿⣿⣿⣿⣿⣿
⣿⣿⣿⣿⣿⣿⣿⣿⣿⣿⣉⠉⠻⣿⡟⠃⠀⠀⠀⣾⣿⣿⣿⣿⠿⣿⣿⣿⣿⡇
⣿⣿⣿⣿⣿⣏⣹⣿⣿⣿⡏⠀⠀⠛⠁⠀⠀⠀⠀⣿⠛⣿⣏⡉⠀⣿⣿⣿⣿⠁
⣿⣿⣿⡇⠉⠉⠉⠉⠉⠉⠀⠀⠀⠀⠀⠀⠀⠀⠀⠉⠙⠛⠻⣤⣴⣿⣿⣿⣿⠀
⣿⣿⣿⡇⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠘⣿⣿⣿⣿⣿⠀
⣿⣿⣿⣿⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⢀⣀⠀⠀⠀⠀⢀⣿⣿⣿⣿⡏⠀
⣿⣿⣿⣿⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠐⠚⠛⠁⠀⠀⠀⠀⣼⣿⣿⣿⣿⠁⠀
⣿⣿⣿⣿⡄⠀⠀⠀⠀⠀⠀⠀⠀⢀⣀⣀⣀⠀⠀⠀⠀⠀⣰⣿⣿⣿⣿⡇⠀⠀
⣿⣿⣿⣿⣇⠀⠀⠀⠀⠀⠀⠀⠀⠀⢀⡀⠀⠀⠀⠀⠀⣰⣿⣿⠻⣿⡿⠁⠀⠀
⣿⣿⣿⣿⣿⣿⣦⣄⡀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⣠⣾⣿⣿⣿⠀⠈⠁⠀⠀⠀
⣿⣿⣿⣿⣿⣿⣿⣿⣿⣷⣦⣄⡀⠀⠀⠀⣀⣤⣾⣿⣿⣿⣿⡇⠀⠀⠀⠀⠀⠀
⣿⣿⣿⣿⣿⣿⣿⣿⣿⣿⣿⣿⣿⣿⣿⣿⣿⣿⣿⣿⣿⣿⣿⡇

Other than some of the official servers preforming a little oddly, I find the comments claiming that Left 4 Dead 2 now overshadows its predecessor, given the L4D1 campaigns are available in L4D2, to be a little dismissive, and almost disingenuous to what L4D1 does in its scope.

Left 4 Dead 1 is much more thematically horrific, providing an ambiance that's hard to replicate in a game of its nature. I find you have to play L4D1 a little more calculative-ly than L4D2, given you aren't provided as crazy of a kit as you are in L4D2, adding to the heavy horrific air that clouds over L4D1s campaigns. I love the audio design, the OSTs gentle but progressive sound keeping a tempo as you struggle, versus L4D2 where you slaughter. A lot more things in L4D1 have this sort of weight, on your senses, and in the gameplay itself, giving it some differences to the pacing and tempo that L4D2 has.

There's a mountain of love and polish that's loud and bluntly present in this game. From how infected behave and react to the environment, to the voice acting and how expressive and lively the cast are whilst fighting perilously against hordes upon hordes of common and special infected, to the various 'chemistries' (I'm inventing a word just for this), the cast themselves share between one another... I could ramble on and on. The map design feels like it was actively played and transformed, with L4D1 having my favorite finale out of all the campaigns, being Blood Harvest.

I'd argue to anyone, even on its own or if you've played Left 4 Dead 2, that Left 4 Dead 1 is a must. It doesn't overstay its welcome anywhere, and is a showcase of a Valve that once experimented with unique ways to evolve their own tech, and formulas, in a focused, condensed, tense and terrifying experience that feels fresh to play decades later.