Good point, but it must be remembered how much Valve promised. They were even promising "Meet the Survivors" videos. I believe I read one interview where it was stated that they hope to be bringing out new campaigns in the first few moths after release, (which I might add was part of their defence for "only having four campaigns" in the initial product.)
Also L4D does have a dedicated team - Valve South. Of course this team is currently working on L4D2 so therein lies the differece.
As I understand it Valve simply promised more than they could deliver. I mean that's fine, there's nothing worse than me bitching and moaning abouit how I'm not going to get "my" free stuff that I deserve well... just 'cause. However Valve made promises and those promises meant that people were more inclined to buy the game. Not me, I was going to buy it anyway but others who maybe thought, "this looks okay but only four campaigns? Only five main weapons?" and were made to think, "oh but Valve said they'd be adding more content almost immediately so I'll buy this," it amounts to a certain degree of false advertising.
Valve may or may not continue adding more to L4D, they said they would but updates have been fairly few and far between for a game that was "almost immediately" going to start getting some serious add-ons. I think adding a new simplistic game mode, (survival,) and a mini-campaign, (Crash,) is a start, hardly what was promised but whatever. Adding Death Toll and Dead Air to Versus doesn't count as an add-on as they should have been Versus mode campaigns at the very beginning.
I still think Valve screwed up. They promised too much and I don't see them delivering anything comparable to what they said they would do either now or after L4D2 is released. It's forgivable but I can understand why people are a tad miffed.