The biggest complaint with Left 4 Dead is that it only has 4 levels which can be completed within about 4 or 5 hours max. Don't worry though, Valve is ready to deliver new content ASAP for the PC and Xbox 360 versions of Left 4 Dead just as they did for Team Fortress 2 on the PC.

On the latest Kotaku podcast(Via shacknews)Valve writer Chet Faliszek said that the new content should be announced soon. To be specific he said "before Christmas" on the podcast. This is a classic case of Valve time.

Rest assured that new content is coming and as soon as details are available (hopefully that's soon) they will be posted here.

Source