Well, I bought the DLC, and it's good, but not THAT good. I'm not sure that valve realise that the nostalgia the L4D2 players feel towards L4D1 isn't just because of the characters. L4D2 really lacks the atmosphere of the first game, mainly down to being too hectice in comparison. By throwing so many special infected at the player, it's become somewhat of a generic action game. The Passing's campaign was a step in the right direction (well, at points it was anyway), but the relative openness of the L4D 2 maps takes away any feeling of claustraphobia that the original had. Plus, the southernifying of the music really doesn't help.
What is really needed rather than just throwing the original survivors into the game (although that is very welcome), is an overhaul of the AI director, to make it play for tension, much like the 1st game. In L4D, I often found myself approaching a doorway only for it to explode as a horde storms through. And hordes seemed scary back then. Now, in L4D2, there are so many zombies all over the place that the horde has lost all meaning, and the AI director throws them in at pretty random times and locations.
Plus, the Tank was actually scary back when the game didn't throw them at you all the time. Oh, and the Witch pretty much loses any effect at all when the maps make it waaay to easy to go around her.