Because of the short window from Left 4 Dead 1, it could have waited.
I'm going to divide my opinions of different aspects of the games like so:
Graphics:
Left 4 Dead 2's graphics essentially trump the original in many ways, from making pipe bomb explosions more realistic, to the way the zombies appear. The new skins of the Special Infected look especially sinister, making players aware that the virus has indeed mutated and things are changing dramatically. Even the guns and available items for the Survivor's have changed- from the original shotguns and mac's to the newer items like grenade launchers.
Sound:
Disappointing, the Southern music cues were a nice touch to the game, but for some reason it just seems out of place. No more exists the creepy, eerie feeling that L4D1 gave us, with exception to Hard Rain (not really the music in hard rain, the storm sounds). However, the zombie sounds are respectfully interesting, especially with the Charger and Jockey. The weapon sounds have changed as well, though seemingly more realistic, they sound very different from L4D1's weapons.
Difficulty:
On the Easiest difficulty, it's still possible for at least 1 or 2 teammates to be incapacitated or killed by the zombies, as opposed to the very soft nature of L4D1's easy setting. The possibility of a Tank appearing has been increased, along with the chance of a Hunter re-pouncing his victims after being shoved off. If you're looking for a walk in the park, this is NOT your game to fool around. Lastly, it appears that more Tanks than before spawn at a finale (estimated at 1 or 2 more).
(Actual scenario: I recently played L4D2 on Expert, and was doing rather well, but my team was low on health. I turn a corner, and sure enough, a Tank spawns in front of me and ends it.)
Gameplay:
Sometimes good, sometimes confusing. There have been loads of discovered glitches since L4D2's release, and it seems like people are looking for more. The game itself plays well in some aspects, but elements (like melee combat) feel almost overpowered. In a Versus match, an Infected team can be at a huge disadvantage to strong melee-based Survivors.
The Survivors:
It looks like no one has really observed the Survivors themselves in retrospect, so here's what I think.
Nick: Thinks he's a bad boy, then turns soft for his teammates. It's been done a thousand times Valve.. why recycle this formula? In general he's not a bad character, but ranks about second to Ellis.
Rochelle: No real character to her, with exception to the intro video. Rochelle doesn't try to stand out much from the group, and as such, ranks lowly for personality.
Ellis: Goofy, and not too bright, but a generally realistic and life-filled character. I've ranked Ellis number one because he's the most believable and interesting character in the game.
Coach: Doesn't really act like much of a leader at times, just tries to be. He's kind of a smart-aleck a lot of the time, and others he hardly says anything memorable.
Overall:
Graphics: 9/10
Sound: 5/10
Difficulty: 5/10
Gameplay: 6/10 (Challenging for unfamiliar players, just right for hardened veterans)
Characters: 3/10 (Not really realistic enough)
Summary:
While Valve attempts to create new scenarios by introducing new zombies to the fray, including new situations, it almost collapses on itself because of difficulty, lack of real change, and static characters.