There are many schools of thought as to how a map can be made more dramatic. All of them have their strong points and weak points. I personally like to add what I call a "movie moments" to my maps.
These are often known as "set pieces" in the film business. A self contained scene where an exciting concept is brought to life to wow the audience. Think of the cave troll fight in The Fellowship Of The Ring or the elevated train fight in Spiderman 2.
OK this is kinda cruel but it's a perfect movie moment. He'll drop before you can reach him and there's nothing you can do!
Within game design, bringing these concepts to life often is mixed up with the player having to perform some kind of action and be somehow involved in what's going on. Personally I disagree with this. The player has enough to do without adding some complex actions that have nothing to do with the regular game play. These events should happen around the player. They can be triggered by a player action or not but the actual event itself should occur despite of the player. Their only requirement being to possibly get out of the way of the event.
A good example of a movie moment occurs towards the beginning of my map. The players are on a small ledge on the side of a building. A helicopter comes in close to tell them to make their way to the roof and he'll pick them up. At that moment the helicopter veers to the right into an unseen power cable. The chopper veers erratically above the players and smashes into the side of the building above them. The wrecked chopper then falls off the edge and narrowly misses our team taking out a path obstruction in front of them as it goes and allowing them to continue.
This movie moment serves several purposes.
As this occurs at the beginning of my map I hope that this gets the players attention. Like the clap of thunder at the beginning of a shakespear play, always start with a bang. George Lucas knows this well as the huge Star Wars logo appears and the main theme roars in your ears. Your in my world now bitches!
In addition this event will hopefully let the player know to expect the unexpected. It puts them on edge and will make them pay more attention whilst playing.
Now, the challenge in Left 4 Dead is that there are very few other characters who can intiate or take part in these events. You have players movements and actions. You have hoards of infected whose actions are managed by the director (I.e. are uncontrolable in a movie moment) and you have unseen drivers and pilots represented by the movements of their vehicles. So your left with the environment itself (e.g. a strong wind, earthquake, fire) and the players actions within it to create your really dramatic scenes.
The one thing to be careful of within these movie moments is taking too much control away from the player. In my earlier blog I mentioned how much I admired half life 1 for doing away with cut scenes. Cut scenes are evil because they normally show you some fantastic action scene and the player is left thinking " why the hell couldn't I have played through that. That looked like the coolest thing in the game!".
For example... Never kill a boss off with a movie moment or rob the player of some fun.
These movie moments are key elements to making your map memorable and setting it apart from the crowd. Use them wisely and sparingly but spend as much time on them as you can as these are the experiences that the player will walk away with and hopefully want to come back and experience again.