I like slightly unconventional dress and general look. I especially like the T-shirt & combats combo. If she has spiked red hair and specs, then I'm officially in love.
Then again, a nice officey outfit can be really sexy. So long as it's not too smart. I do like it when women don't look like they've spent HOURS getting ready. I find it a turn-off seeing women groomed to within an inch of their lives, where it looks like every individual hair has been carefully positioned.
Pristine white clothes are unattractive too.