Man i love L4D and i have nearly all the achievements this is the ones i have left:
Complete the campaign using only pistols (Yeah, i cant resist T.T)
Complete a campaign without a Boomer vomit on you
Complete a campaign in Expert
Complete all campaigns in Expert
Kill a large and ridiculous amount of zombies (Cant remember)
Complete a campaign without getting damaged by a Tank,Boomer,Hunter or Smoker (Impossible >_<)
Kill 10k Zombies with the Turret
Complete a campaign without getting any FF case
Anyone didnt get damaged when the Rescue Vehicle appears (Its very difficult, because many Specials Zombies appear and appear)
Thats all :P