I'm just reviewing offline, and offline with 1 or 2 players this game isn't too fun. Wish they put more effort in game.
First of all there isn't much of a story, the game starts some time into the outbreak in New Orleans without proper intro and it doesn't get any better. There is some dialogue between the 4 survivors, but it's just small talk and conversation on how to escape. The game works just fine without robust story, but there could be something more to get the player involved. The game is divided into 5 varied chapters, players can pick any of them from the start and the episodes are designed to be played from start to end in one sitting. No checkpoints that save your progress, although you can choose a generic checkpoint, which don't count for Achievements. It's obviously built for accessible co-op instead of any thought on focused story. Each episode only lasts for about an hour, making this barely a 5 hour game. There is a survival mode to boost longevity, but it feels pretty much the same being set in areas from the story.
Worse issues are in the actual gameplay though. Firearms as well as melee weapons lack punch, which makes killing zombies unsatisfying. Shotgun has no kickback and the enemy just falls down as you shoot. Moreover the physics and A.I. of enemies seems very simple, the common zombies just run at you and circle around to beat you up with shoddy hit-detection, special infected also have just 1 albeit dangerous trick in their arsenal. A.I. teammates do their job decently most of the time, but occassionally they fail (for example walk into an open fire and die without anyone being able to help). Especially during the final battles at the end of episodes they usually die and I end up being the only survivor to continue to the next episode, only to have those teammates come back to life like nothing happened.
The Director making every playthrough unique is a nice idea, but it won't be long before you notice the pattern and realize how predictable it all is. You walk some distance, music starts to play and a swarm of zombies spiced with some mutants appear all around. You deal with them and soon it happens again, and again, and again. Though it is a wonderful feeling to loot the apartments with a couple of zombies hiding in the bathrooms or closets, I wish there was more of that instead of the large scale outdoor battles. Each chapter also has a bit more interesting Director effects such as rain or closing off paths of a courtyard, but they're miniscule things.
Summary: Left 4 Dead is obviously made for full 4-player co-op. With 1 or 2 players it isn't all that enjoyable. The game isn't good enough to make use of the "infinite" replayability, during the first playthrough you've seen it all, repeated to a tenfold. Combat lacks feedback and A.I. is unstable. For once random selfish jerk human players online seem to fit the style of the game, strange people only looking after themselves during a zombie outbreak adds to the immersion.
Wishes: A proper and longer story wouldn't need to damage the accessibility. Aside from that completely overhaul the engine - weapons need to have punch and preferably blast zombies to pieces, and hit-detection needs major improvements. On the contrary common zombies should do more than smack the players, a lone zombie should be a bit more fearsome than they are now. The game could also use more dark levels and enclosed buildings. Most important is reducing repetition and make Director have a much bigger pool of possible actions.