Left 4 Dead is a coherent world game in which there is no specific time flow throughout each level. Playing one level for several hours or just several minutes will not change whether or not there is daylight in the next level. That being said, you do sometimes have to perform a task or defend a building within an allotted amount of time to advance on to the next level.
One of my many talents (haha not really) is my super amazing ability to obsess really hard over things I really like. I will scour an entire wiki, read any blogs, scan through discussions, and stare at fanart for hours at a time upon a newly discovered interest. My intense obsession for Left 4 Dead did not come to full fruition until Left 4 Dead 2 came out and introduced my two favorite characters, Nick and Ellis, along with three new Special Infected; the Jockey, the Charger, and the Spitter. Upon playing through the game in its entirety, I took to the internet to find more.
Now, my brother is a much better gamer than I am and he also enjoys a good game of L4D, but he's also one of those gamers who isn't really that interested in why he's there, he just wants to get good stats and win. Playing together is often frustrating because I want to watch the cut scenes (again, for the millionth time) and he wants to skip them. I also enjoy exploring and reading all the interesting things that "other people" have written on the walls in some buildings, where as he finds the most direct path to the safe house and just goes.
So, it came as no shock to me that, even though he has actually played many more hours of the game and is a lot better at it than I, that I actually know more about it, simply because I payed attention to dialog and I read a few things online.
For instance, I'm not sure how many players know this because I'm not sure that it was mentioned in the game (I have yet to play the newest campaigns), but in a comic by Valve (creators of L4D) that was released just before the campaign "The Sacrifice" it was stated that the immunity gene is passed down from the fathers side of the family, meaning poor Zoey had not had to kill her father when he was bitten. My brother didn't know this.
Another interesting tid-bit is that in the earliest test version of L4D there was a special infected known as the Screamer, who looked like an escape mental patient, complete with a straight jacket. Unlike the other SI, he ran from the heroes, but not before letting out a blood-curdling scream that would attract the horde. He didn't make it into the final product, however, because the testers found that he was too difficult to find and kill before completely ruining your day. Most of his concept was reused, however. His screams were given to the Hunter, his ability to attract the Horde was given to the Boomer's bile attack, his attribute of standing idly in one place until alerted was given to the Witch, his maniac cackle was used for the Jockey, and his need to run from the survivors was given to the Fallen Survivor (uncommon infected).
Left 4 Dead is on it's own tells an interesting story about survivors and the infection that is tearing the world apart, but it takes a little digging outside the game to uncover lots of neat little extras.