Rubin's reasons why products are hard to use

Brief summary of reasons from pp. 4-8 (pp. 6-12, new edition).

1. Product development has a focus on the machine and the system and not on the end user.
2. The target audience changes.
3. Developers underestimate the difficulty of designing usable systems and treat their design as "common sense."
4. The specialized teams used for product design and development fail to have their contributions successfully integrated.
5. Developers fail to distinguish between design and implementation.