Brains like familiarity, but they get bored. They are genetically programmed to want to discover new patterns. You don't want it too new because that seems dangerous. You want it somewhat familiar and somewhat new. Think of music. The best music has some kind of essence of things you can recognize: a normal beat, harmonies, and melodic phrases, but you don't want to hear the same old, same old. You want something that's slightly jarring, and a little bit clever. The newness matters more than any other particular aspect of the aesthetic value. You want newness combined with cleverness. Somehow new and old at the same time gives the best design. If a design is so new that people can't relate to it, then they reject it, even if they could theoretically learn how to use it because it's very clever. Styles are like this in general; if you have a new style for clothing, generally you don't want it to be too crazy. You want it to be just slightly different, enough that people say, "Oh, that's cool." It's built into the human brain. We want familiarity, we want to be able to learn how to use it, but we also want some newness to it, and that's what makes us excited about it. [As cited in Bill Moggridge, Designing Interactions, page 234 - no further details (page numbers, etc) in the reference].