I'm definitely going to get l4d2, although now seeing the quote dug up from Gabe Newell, it is a little bad to read...
"So each time we've released one of those for Team Fortress 2 we've seen about a 20% increase in the number of people who are playing online. And that number is really important because it determines how many community created maps there are, how many servers are running, and so on. So we'll do the same thing with Left 4 Dead where we'll have the initial release and then we'll release more movies, more characters, more weapons, unlockables, achievements, because that's the way you continue to grow a community over time."
Plans change and all that.