Newell was asked about support for Left 4 Dead and Left 4 Dead 2, relative to other Valve titles, and the chief offered that "Team Fortress is the benchmark and that's the level we intend to bring all products to.
We're going through transition, which I think is interesting. We call it "entertainment as a service" and Team Fortress is furthest along, in terms of rapid regular updates, and moving into movies with the 'Meet the Team' shorts, as well as we're starting to do some stuff with comics. So, Team Fortress is really the model that the rest of our products are gonna be following as we move ahead.
As to why he honored modder Joe W-A's request to get down to Brisbane and see his Left 4 Dead campaign:
We recognize that we really are part of this community. We talk about it a lot, that we're not up here and the community's down here, and the games snowflake in. We really are part of it. We look to that community to see what we should do. ... It was pretty clear that this was one of those things that the community fell in love with the idea. They wanted us to come down here and meet with Joe and give him feedback. And in our experience, it's been very bad idea when we get such a clear signal from pc gaming community, if we ignore it.
Fly Gabe Newell - Gabe Q&A - Part One [YouTube, from Joe W-A]