The underlying gameplay of Left 4 Dead is old-school simple: Bad guys rush you and you gun them down. If this was all there was to the game, it would be briefly amusing but ultimately dull.
But the meat of the game is with the cooperative play and the fact that you need your teammates in order to even have a chance at survival. That interdependency arises from the effects of the special infected. They are designed to break your formation, confuse your strategy, and force you to adapt.
The boomer is one such agent of chaos. If you get near him, he pukes on you. If you shoot him, he explodes and gets his goop all over everyone nearby. Once someone is covered, they're blinded for several seconds and the stench attracts huge numbers of common infected to swarm the now-helpless victim. Their teammates can help them, but the thing about boomer victims is that they tend to freak out when they're surrounded by zombies and all they see is green soup. Their desperate panic fire is likely to hit you if you go to their aid. Does the victim hold still and trust their teammates to aid them? Do their teammates rush in and trust their vomit-glazed compatriot to hold his fire? A few moments earlier everyone was back-to-back and working together, and now everyone is finding out just who they can really trust with their life. It's insidious.
The boomer is a huge target, he's a slow mover, and his gurgling and belching can be heard long before he gets to you. It's a credit to the AI that the game is so good at getting this guy to ambush four alert human players despite these disadvantages.
Now if you'll excuse me, I think I'm going to be sick.
Shamus Young is a programmer and writer by trade, videogame nitpicker by inclination. If you have the patience for more of his ramblings, they can be found at ShamusYoung.com.