Valve's seems quite serious about releasing Left 4 Dead 2's first expansion, The Passing, this week... or, according to today's update, "soon". So it's probably high time we find out what those new game modes are.
While Valve still isn't telling, officially, it's hinting at new changes to the game in a new update at the official Left 4 Dead blog. They warn that things could get spoiler-y, if you're overly sensitive to these types of things, so consider it noted.
One new mode may be called "Mutations". Hey, it's one of the Achievements, requiring that players play through six Mutations to secure the Mutant Overlord achievement. And unfortunately, that's the extent of the detail they provide. Another may involve the opening of foot lockers - not to provide zombies with quality footwear, but to, presumably, find well-stocked weapons caches.
Read on for more potentially spoiler filled details, then hurry up and wait for more info later this week.
Passing an Achievement [Left 4 Dead Blog]