Left 4 Dead: This game is really the poster child for dynamic content because that's its raison d'etre. Valve wanted to explore dynamic content in the form of their "AI Director". They built four short "campaigns" but then put the real work into a hugely varied and responsive system of spawns, spawn points, and spawn rates.
Eh, that's not really how it's done.
All infected can only spawn in places the survivors can not see them. Whether this be behind a tree, on the other side of a wall, or what have you... it's essentially the same thing.
There are no spawn points or system of spawns. You don't place spawn points, you create areas where the survivors can not see the infected. Each special infected is on a 20 second timer, tank and witch excluded, and each horde is dictated by a few factors:
1. Average player movement speed. The faster, the less likely a horde will come.
2. Boomer bile. I forget the number, but a designated horde will come at you every time. Depending on the layout of the map (read: where the survivors are biled) the infected may come at a trickle or may come in a blob. It depends on if they can all spawn in one big area at once or if they have to spawn in several smaller areas.
3. The amount of time that has passed since you had a horde on that level. The longer, the more likely a horde will come.
4. Rate at which you have killed infected. The higher, the less likely a horde will come.
That's it. This is why maps like BH4 can be beaten without a single horde attacking you and with a small smattering of infected (maybe 20-30 on the entire level). The only thing that slows you down in that level would be a special infected (tank and witch included). There are just enough zombies to kill so you don't get a horde called on you, you move fast enough so a the AI director doesn't call a horde, the special infected can't keep up with you, and the level is short enough to where you the maximum time allowed for the AI director to hold back isn't even closely touched.
The only thing that is truly random about the director are the weapon and item placements... and even then, it's not really that random. Other than the designated weapon and item placements (medpack in ambulances, shotgun in the quicky mart of DT 4, the molotovs on DT1), the other placement for items are generally just a choice between "none, this, or that".
L4D really isn't dynamic. It has a specific set of rules that depends mostly on you the player. Timers exist to attempt to limit the player while level design is there to attempt to slow you down so the AI director can spawn things on you while random exists to attempt to make the player believe that it's all random.
Valve did a good enough job obscuring and advertising the random part of their game so most people don't recognize the obvious patterns, timers, and other things that make up the real meat of the game.
The only real astonishing thing about L4D is the combination of the AI director and his ability to make sub-par level design (tons of empty, useless rooms most of which don't have the item spawn triggers that the director uses to determine if an item is placed in that spot or not) an enjoyable experience. The more useless rooms you have, the more the player explores them, the more he gets attacked (seemingly randomly), the more the player feels like he has to explore them to find the items he needs to destroy the hordes that come, the more hordes that come...
Of course, people exploit the director to hell once they understand how it works.
