You did it! You achieved the goal of playing a ton of Left 4 Dead 2, in sufficient numbers to unlock more Left 4 Dead 2. Valve challenged the community to get the "Stream Crosser" achievement 20,000 times by completing the Cold Stream campaign, and after 24 hours that achievement had been achieved 60,774 times.

As a result, Valve is releasing the L4D2 version of the L4D "Dead Air" DLC early, as promised. The initial announcement said July 22 -- now Valve is saying July 22 at the latest and intends to release the content as soon as it's finished.