Browse the steam forums and get an idea of how pissed off people are about this.
there are groups and petitions and such bitching at valve. I'm completely disappointed in valve with this. They promised updates and stuff just like TF2, new weapons, new players, new infected, and they kept that promise, but just decided to ass a $40-50 price tag with it. It's not a new game or a sequel, it's an expansion pack I'm sure (amount of content-wise). It makes no sense and just pisses me off, because now you have the community divided, L4D and L4D2. If they would of just released updates and such it would be way better. You could have 8 characters to choose from, and more weapon choices.
Also there are some idiots that think the last batch of content valve gave us was some huge deal. They gave us a new mode, with a special map for that mode. And they gave us VS on maps... that should of had vs when the game was released
Either way, I'll most likely end up buying it unless the demo is a complete disappointment.