My definition of attractive generally means harmonious facial features (nothing too big or too small, positioned visibly weirdly), clear skin, symmetry, sexual dimorphism, class 1 bite with good teeth well aligned with the face and forward-grown jaw structure. Id say there is a specific "looks like an entrepreneur" element to it where these people tend to have a somewhat friendly, vaguely supercilious, clean cut, non-distinctive look. However a preponderance of the former objectively attractive features is hard to ignore compared to the people you see on the street, at Starbucks, at the Post Office, etc. I do also agree grooming and the effects of a good diet/exercise routine which correlates with the strata of educated middle upper class people who apply to YC is a factor too.