Usability for Nerds/Avoid anthropomorphic interfaces
Some designers are tempted to make user interfaces look like cartoon characters or communicate with the user in a human dialog style, e.g. when the computer says "Tell me what you want me to do". This may look cute, but experiences with anthropomorphic user interfaces are generally not very good. For example, Microsoft's animated paper clip called Office Assistant was so unpopular that they had to remove it. Many people found it annoying, and some even considered it an insult to their intelligence.
People have an extraordinary tendency to attribute intelligence and personality to dead things. The more a machine responds or moves in a natural way, the more will people tend to believe that it has intelligence and emotions. (A pioneering example was the Senster, which people believed was intelligent because of its natural-like movements, even though it didn't look like anything but a machine). People will have unreasonably high expectations of the social intelligence of a machine that looks like or behaves like a living creature. They will unavoidably be disappointed when the machine doesn't meet their expectations.
There is also an ethical problem in making a machine behave so that people get the impression that it has emotions and intelligence. If you make a robot that behaves too human-like, you are likely to get angry reactions from the parents of children who have interacted with the robot in the belief that it had some kind of life or emotions.