Best trash is thrah that serves a function. For example, the trash before Durumu teaches you the mechanics of the fight. The fog beasts that reveal adds when they die, the eye thingies that create a mini-maze that you have to run. That's good trash.
Bad trash: Piles of mindless trash that serves no other purpose but stretching the raid out, with (almost) no reward. Mount Hyjal comes to mind with the silly attack waves.