Yeah, I often break shooters into two categories, the unscripted ones, such as L4D, or Far Cry or Crysis. And scripted ones, such as Half Life or Call of Duty. The director works so well for L4D but Half Life is just fine without random encounters. Since L4D is designed to be played over and over it makes sense for people not to know that they can run forward to a certain place, then duck, turn and fight zombies, know where the witch is everytime... Half Life is about set pieces, like defending a house or storming a gun emplacement... how would it be if there was nothing to defend against, or you attacked a bunker filled with hundreds of combine (and a witch). Or indeed a bunker with no combine.