I LOOVE these games. I still play lfd2 alot. I totally agree though with most of the sentiments here: the second one is better for gameplay, but the first has better ambiance. the darkness, and "feeling of dread" are totally not there in the second one most of the time. the levels are too open and bright, but perhaps thats what they wanted, as they wanted a different feel for the art direction maybe? I dunno.
interesting tidbit: the boomer was origonally two different specials: the leaker, and the screamer. the leaker was like the boomer, but he just leaked the bile on you if you shot him. the screamer was an infected in a straight jacket, if he saw you, hed run away and scream, attracting the hoarde. the devs found the screamer to be OP, since finding the screamer while fighting the hoarde was almost impossible. they then just rolled the two into the boomer instead.
also, the games working title when it was in the concept stages was "terrorstrike" as in "counterstrike" (another one of Valve's games)