The "Mom" Test
Long ago (like 1996ish) I had a series of arguments on the Future Culture mailing list about why Linux was unusable to the masses. My litmus test: could you put someone's grandmother in front of a Linux box and have her write and print a letter to her grandchild without help or nervous breakdown.
While apparently an actual grandmother wasn't used, current Linux usability testing seems to be
moving in the right direction. [link via Slashdot]