One of the best things I've read in past relating to this topic dealt with the playtesting that Valve held for (I think) Half Life 2. If anyone could supply me with my lost source, it would be greatly appreciated.

They would sit a wide range of users down in front of a new level or scene, and silently watch them play, with no help, and with analytics running in the background. If they noticed players taking too long on certain puzzles, dying too often from certain monsters, or just not grasping the expected flow of events, there was further work to be done; likewise, if players blew through a level or puzzle too quickly, didn't have any troubles with a "boss", there was also work to be done.