Left 4 Dead 2 came out a YEAR after the first plus, the new zombies are so sick.
The new special infected — the Charger, the Spitter, the Jockey — all fill distinct roles that complement the original roster rather than replacing it, and the expanded campaign lineup includes some of the best Left 4 Dead levels in either game (The Parish's bridge finale and Dark Carnival are the obvious highlights). Valve also eventually added the original's campaigns to L4D2, effectively making it the definitive version of both games.
It's the same core design as the first game with more of everything that worked, which is the correct approach for a sequel when the core is already excellent. If you're going to play one Left 4 Dead, it's this one — the original's tighter economy is interesting, but L4D2's breadth wins.