Seed Magazine has an article on recent research published in Psychological Science that suggests that average faces are more attractive because they are easier for the brain to process.
The image on the right (go to the article for a bigger version) is a composite of a number of different female faces rated as attractive.
However, an average of all sorts of faces also tends to be attractive, as demonstrated by a page at the University of Regensburg (which also has an image of an hot average man as well).
In the Psychological Science article (pdf) the research team, led by Prof Piotr Winkielman, asked people to judge the attractiveness of shapes and dot patterns. Participants were more likely to judge the most average patterns as attractive.
In a further experiment, they used the same technique for faces and found the same result.
The researchers argue that the reason we prefer average faces is because the brain creates an idea of a ‘prototype’ face, based on the average of all the faces we have seen. Attractive faces are the ones that best match this prototype because they require less processing to match and recognise.