I am going to give reasons why l4d2 is better than the first and vice versa. I will NOT compare atmosphere or characters as they were imported into the second game also I will NOT include the community of either game as an upside.
Left 4 Dead: + No Jockeys + Balanced versus + Feet + Infinite shove (I like it) + Stronger tank + Smoker insta-kills + Higher FPS + No chaos crescendos (Gauntlets that cannot be turned off) + Last Stand + Coordinated special infected + More skins + more detailed skins for commons + Ragdolls
Left 4 Dead 2: + Spitter and Charger + Melee weapons + Mutations + New campaigns (With old ones) + Workshop + New gamemodes (But nobody plays those) + More weapons + Stronger smoker + Gauntlet crescendos + Unpredictable director + Uncommon infected + Adrenaline rods
I like both games equally. So I wanted to say why I enjoyed them both. If you think I should add something here, please let me know. Also feel free to judge my choices if you will.