Rock Paper Shotgun has a podcast interview with some Valve dude. I haven't listened, but someone wrote notes while listening:
* L4D2 started production immediately after the first was released
* new mutations, southern zombies, New Orleans locale
* takes place shortly after events of L4D
* "Charger" special infected, half tank/regular zombie. Runs at survivors, can grab and smash them about
* more gruesome zombies/deaths, destroyable limbs
* melee weapons like frying pan, axe, baseball bat have different effects
* "uncommmon" common infected: hazmat zombies immune to fire, random mutations throughout
* daytime maps, different types of zombies at night/day
* witch can wander around in random paths, still scary strong
* L4D will still be updated (until or after L4D2 I'm not sure)
* custom maps for L4D will work with L4D2 instantly
* lots of new weapons, more than just five
* team wanted to add more than just new maps, reasoning for sequel
* SDK and matchmaking will be released after E3
* wanted a clean slate to work with L4D2, old characters won't return. it's about the WORLD, not the PEOPLE
Looks like the name "Left 4 Dead" was actually referencing the game itself. Released it, and started work on the "sequel"...which is SO DIFFERENT that maps made for the first game will work in the second. They just couldn't POSSIBLY update the original game.