Don't make me think
In the interview linked below, usability expert Tim Altom provides a heuristic for usability testing. When testing he invokes the simple philosophy he picked up from Steve Krug's bestselling book on usability, Don't Make Me Think.
More in this interview/article on usability testing.
"If anything in the interface will make a user pause and have to think, it should be flagged for possible change," Tim said. "Contrary to popular belief, nobody really wants to think while working with a tool. Thinking is hard work in itself, and should reserved for the job, not for the tools."
More in this interview/article on usability testing.