Grandmaw Testing.
This is my term for the conceptual question, "Can your grandma, who's never used a computer beyond email and checking www.cutecats.com, use it?(Assuming that she has the real-world knowledge to use that particular app)".
Everything common should be obvious; nothing should be black box magic with side effects. Uncommon things should be accessible in a common format that the user has used before.
Clear labeling, clear route to a help file, clear actions with clear effects.
If Grandma can't use your Paint program, you need to really think about your UI.
