Well, Valve has expanded TF2 for free pretty considerably.. but whether or not they plan the same for Left 4 Dead remains to be seen.
Looking at the activity in the forums I think this game will be a massive hit. The Steam forum for it sometimes has about 2 pages of posts that are within 10 minutes of each other.
Not only that this is Valve we are talking about, anyone who has a steam game only needs to look at the update history for their games, they have a superb history of improving on great games. They have said they are increasing the amount they are investing in the game in terms of advertisement, if that doesn't follow through to after sales development I would be stunned.
Even if they didn't do an iota of work imagine how much the modding community is going to do. People are already creating their own maps for it before it is out, I'm even tempted to and I haven't created a map since Quake 1
I'm starting to sound like a fanboy but the demo has me very excited for the game.