Left4Dead is much more gritty, scary, and dark. The characters respond in a very smart manner when they are either reloading, picking up a weapon, locating something, etc. Levels are very claustrophobic causing you to panic when you are being chased down a dark hallway when a witch on fire is chasing you (or a zombie or twenty). You got your Boomer, Smoker, Tank, Witch, and Hunter. Left4Dead is how a real zombie apocalypse is supposed to play in a video game. This iteration of the game does not have the stutter bug that comes with Left 4 Dead 2.
Left4Dead 2 is comical but more gory. The pink mist you get in L4D1 with the pipebomb. Now you get body parts flying everywhere. The genious of the AI on your team has been significantly reduced. They no longer respond with "pills here" but with "I found a thingy here and i's gonna eat it cause im stoopid". The levels have more open areas which leads to a lesser chance of being cornered and much less scares in the game. You got the same 5 special infected plus the Spitter, Jockey (no reason for this if there's a hunter), and Charger (no reason for this if there's a tank). Witches can now move around now, which makes them much less dangerous. MUCH less dangerous. Left4Dead2 seems to be a satire of the original game. Left 4 Dead 2 also comes with an annoying bug that causes the "Multicore Rendering" feature to stutter and potentially crash your game. Valve really needs to fix this.
I have both games. I can tell you Left4Dead1 is much better than Left4Dead2. If you want more skilled players to play with, L4D1 is your game. If you want more people to play with, L4D2 is your game. If you want to panic because the zombies got you, play Left4Dead1. If you want to panic because the special infected got you, play Left4Dead2.
Sorry for the bump but this explicitly needed to be clarified.