Yes, Left 4 Dead 2's release is shockingly close to the release of the original Left 4 Dead. And yes, Left 4 Dead 2 plays very similarly to the original Left 4 Dead. But there's a reason for both of those things.
Left 4 Dead 2 is arriving almost a year to the day after the original of the first, partly because the team responsible went to work right away on the sequel, without taking the standard week off to recharge. Much of the core gameplay from Left 4 Dead will be intact in the sequel.