There's an argument to be had that Left 4 Dead is still worth playing, even though it's basically been recreated in its entirety in Left 4 Dead 2. L4D1 is much stronger as a tonal piece - no melee weapons means you have little in the way of comfortable fallback options, and you have to be conscious of every bullet. When you run into a Witch, you're making a much harder decision about how to handle her here. Less variation in gameplay modes means you're simply running for longer stretches of time from set piece to set piece, which adds that tasteful wearying sort of experience you want out of a straight horror game. These are all subtle differences, and chances are good that you'd like both games if you like one. Personally, I find all the maps I've played across both titles much stronger in their L4D2 incarnations, but I can respect someone who holds the opposite opinion.