Blinded by the Boomer
It moves slow as a glacier and is effectively quite weak but the Boomer can be lethal. That's down to its vomitiferous projectile - a spew of gunk that sends all surrounding zombies on a killer beeline right for the poor unfortunate victim covered in the stuff.
See, you could take the game slowly, sensibly and kill enemies methodically, never breaking a sweat. But Left 4 Dead constantly pulls the rug out from under your best plans - especially so when you're hit by Boomer bile, or a dying Boomer explodes messily nearby, blinding you.
You back away desperately, trigger-finger twitching because you know that in seconds you'll be surrounded by zombos. Being blind, of course, you're just as likely to injure your teammates. So then they start to panic. Escalating fear and panic is what Left 4 Dead does best.
The Hunter is on... you!
Contrasting with the sloth of the Boomer, the Hunter is a horribly quick, leaping beast. Generally, you'll have no problem taking it out once you've seen it. The problem is when you don't see the Hunter. Which is often. On one 'last stand' occasion we'd cleared everywhere - we're sure we had - and had turned to return to the group, only for a Hunter to hurl itself onto us from behind, so fast we had no escape. You might sometimes think you're safe in Left 4 Dead, but really you're just waiting to be hunted.