I tried the Left 4 Dead demo on my Xbox 360 last night. Wow! This game rocks.
Many features make it great, but none so much as the unpredictability introduced by the dynamic spawning. Brian and I played the demo for an hour or two last night, and every playthrough felt fresh... despite the environment and goal being exactly the same each time. You never know when or where the next attack will come, how many zombies will come at you, or what types of monsters may join them.
Once, we all died and so started back at the safe point. When we got to the area where before we had been overwhelmed with a huge horde of zombies, this time that area was completely barren. It was eerie. I remember Brian saying, "I've got a bad feeling about this". The areas just beyond were also empty. Then, of course, we got smacked by a couple dozen zombies.
Where you are when particularly nasty strikes come and what condition your team is in from previous fighting can make all the difference in the world. How many directions can enemies come from? Zombies can come from directions you just cleared (which the environment design makes believable, by the way).
Is there a defensible position (like a corner, in a train, or on top of a car) nearby? At one point, we were progressing through a subway train when a Tank appeared outside. When the Tank repeatedly slammed against the side of the train, leaped on and off the roof, I was certain he was too big to get inside. I was wrong. And yes, we all died.
Even reloading is a powerful dynamic in this game. In most FPS games, like Halo, enemies move slowly enough and there are few enough of them that you can run and dodge while reloading to avoid damage. In Left 4 Dead, enemies are quick and they often come in hordes. At exactly what moment you must reload your shotgun or assault rife is sometimes the difference between life and death.
The game adapts to how well or poorly you're doing. Sometimes you'll find extra weapons, ammo, health packs and pain pills. Other times, you'll be sorely disappointed. You can select a degree of difficulty at the beginning, but the game adjusts dynamically within that selected difficulty range.
Brian and I spent the majority of our time trying the demo on Expert difficulty. I don't know how many times we tried to beat that level, but we never reached the safe point (the level's end). On the last try, I was almost within sight of the end. I was the last survivor. All I had to do was run up a small flight of stairs and sprint a short distance to the safe room. But when I entered the stairwell, lo and behold, there was a Hunter waiting for me on the steps. I got a couple shots off before it lunged, but not enough to stop it from making a meal of me. Such is Left 4 Dead.
And that's just the demo: 2 levels and one multiplayer mode. I haven't tried Versus mode, but it sounds great. The full game includes 4-player co-op, 8-player online battles, split-screen, and 20 maps.
Dynamics, dynamics, dynamics. The variety and unpredictability is the most important aspect of this game. Left 4 Dead has quickly moved to the top of my must-buy list.