Kinda not too keen on L4D2. I love L4D and play it nearly daily however this is expansion material. I wouldn't mind shelling $20-$30 for this content but pushing it for full retail price is a bit too much. And Valve should know better, the L4D community didn't really kick off until they started offering the game at half-off ($25) on Steam.
But, even money aside, I'm still dubious about this expansion. The music is corny, the setting is meh (special infected wearing over-alls?), the only character of the new cast I sorta like is the mechanic kid. The original cast is much more charismatic and it has nothing to do with race (I play Louis almost exclusively). They just don't seem very interesting.
One of the things I like about L4D is that it still fits (though very loosely) within the realm of possibility. The zombies are not magical zombies but more like humans infected with a deadly strain of rabies. The guns you use, the way you fight, how you conserve resource "feels" as real as a game can make it feel. Now they are introducing fire ammo, chainsaws and axes and all this other nonsense, pushing the game away from horror shooter and more towards arcade shooter. And now that we get effective melee weapons, the need to conserve ammo is drastically reduced. I don't like any of this one bit.
However, there are rumors that Valve is thinking of including the original L4D in L4D2 for Xbox owners so that they don't have to swap discs in the event they want to play the original levels. If this is the case, then the PC version might truly be an add-on to L4D1. If that is the case and I can use members of the old cast in the new levels then I will be somewhat content.
My other worry is that the playerbase will fragment heavily between those that stick with L4D1 and those that buy L4D2 if L4D2 is a standalone game instead of an expansion. As an expansion, they could make it that L4D1 players can play with L4D2 players so long as they are playing L4D1 maps, modes and characters. As a standalone game, the playerbase will be divided.