Not much of a surprise this is a possibility, the two games do use the same game after all. But Left 4 Dead hasn't even launched and someone's already modding it (probably the demo, anyway.) Here's nine minutes - reloading! - of Left 4 Dead gunplay on TF2's Dustbowl map (slightly customized). Warning, the volume on this sucker is hecka loud for some reason, with tons of - reloading! - gunfire. Left 4 Dead Meets Team Fortress 2's Dustbowl [CS-Nation, thanks AproposOfEverything]
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Anybody play the demo and feel the zombies are too easy?