Well, Left for Dead 1 came out not even a year ago, so this obviously isn't any new spectacular game. So it sounds like they will make you pay a good $30 (knowing Valve) to buy a big pack of maps, skins, few new weapons, and new blood effects. I think that this could have easily been made into a free mod, especially since the SDK just came out.
Left 4 Dead 2 looks sick. I bet you that Left 4 Dead is going to be one big story of random people maybe later on they will tell us how every thing came to be in Left 4 Dead. This is just what i think.