Concerning Left 4 Dead 2, he was prompted on why Valve has chosen to create a sequel so soon after the original L4D release. Newell went over several reasons, ranging from scheduling to talent. The most interesting point he brought up was that directly after L4D's launch, there was still a lot “on the table” that developers and game designers wanted to accomplish. A lot of their goals, he said, came directly from playing L4D themselves. Further, he also commented that Valve doesn't plan on ceasing updates in terms of both bug fixes and content for Left 4 Dead once its successor is available – whether or not developing for both the original and sequel simultaneously will have an impact on quality remains to be seen.
He also talked about limitations of existing hardware platforms, such as the Xbox 360, which apparently make developing additional content for L4D more difficult. Switching topics to Team Fortress 2 and Half Life 2, he addressed concerns about recent updates to TF2. Primarily, he just said that changes are still underway, and that certain decisions – like making drops in TF2 random – will make more sense once trading systems are implemented. Overall, it's a long interview, and well worth the read if you're fan of Valve games.