Traditionally the attraction to intelligence, however it can also be extended to the attraction of proper, full sentences and the use of grammar.
Do I read the Toronto Sun? No, I am a total sapiosexual, I like sentences, paragraphs and grammar.
(adj) A form of sexual orientation characterized by a strong attraction tointelligence in others, often regardless of gender and/or conventional attractiveness.
(n) A sapiosexual person.
"I am sapiosexual. I think geeks and nerds are sexy--I often want to rub my clit against their minds." -Kayar Silkenvoice