L4D1: 9 months to release Survival Mode, add Versus Mode to 2 of the campaigns, and now a 2 map campaign that lasts a projected 30 minutes according to several articles.
L4D2: 11 months to make 5 campaigns with Co-op, Versus, and Survival Modes, new weapons and playable characters, improved AI Director, and new Special Infected.
I like and play L4D as much as the next guy, but this is kind of silly.