Glasses tend to make people look more intellectual, which is a complete turn on for me. If they're styled so that they fit the person well and go with their facial features and overall style, glasses can look really good on a girl. For example, Tina Fey looks much better in glasses than without. The dark, thick rimmed glasses especially seem to do the trick.
This might sound weird but I think girls are pretty sexy in sweatpants, sweatshirt, and glasses. But at the same time I like when she dresses up and doesn't wear them so it really depends on the occuation.