The biggest problems that i tend to see (Score 1)

are when software has multiple, seemingly redundant menu options, and when software behaves in an unexpected and (relatively) unexplained manner. I base these observations on my mother, who has difficulty programming a microwave. She's not by any means an unintelligent person, she's just not technologically inclined, and some of the things that we Geeks absorb and automatically 'get' give her more trouble than my teenage siblings.

I wholeheartedly subscribe to the theory that if my (mother|grandmother) can use it without any more assistance than usual (troubleshooting and the like), it's a generally well-designed product.