Props for making a kickass poll that was actually difficult to choose a response to. I chose crescendo events because I am having the most fun in L4D when shit goes sideways and you're shooting bullets and then changing guns instead of reloading, backpedaling, and then sacrificing your hard won safety to go save your bro. When shit goes crazy, that's when I have the most fun.
I also agree that the night-time urban enviroments in the original game were the most fun, namely the subway, but I also have to attribute a lot of my loss of love for L4D to the fact that after the first game, the scariness goes away, and it never comes back. It becomes where's the best vantage point, where's the best place to shoot the zombies, how can we bring down the tank fast enough, not HOLY CRAP THE ZOMBIES ARE EVERYWHERE. Twitching because you heard something behind you, screwing up in the game because you damn near pissed yourself.
And that will never come back. But, if it's any help, night time urban enviroments with kickass, insane (but still beatable) crescendo events are what made the first game so much fun.