This is a great idea, and one that Valve should have thought of as it makes sense to show the "original" survivors in Left 4 Dead 2's Intro as well as the "new survivors", since you may play as Bill, Francis, Louis, or Zoey as well. Personally, I think the Left 4 Dead introduction is better than the Left 4 Dead 2 introduction as it is a bit creepier since you know the zombies are mutating only 2 weeks after the outbreak began. It was neat just to see this introduction, as I never owned the original game. The only negative I can think of is I had to turn up the volume on my speakers to be able to hear some of the things Bill says- however, if that is anyone's fault it is Valve's isn't it?
This intro explains most of the main gameplay mechanics - witches, tanks, smokers, hunters, pipe bombs, car alarms etc. So it's really worth watching it, even if you've never played the original game.
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L4D1 Intro Plays Before L4d2 Intro Review