I think of attractive as something that attracts me. I am never attracted to the person's looks. I am always attracted to their personality though (if its good that is). So, for me attractions is about their personality and behaviours.
Now, many people think that girls like materialistic, good looking show offs, the flirty type boys that all they do is talk and not do any hard work. But I would not think so. Most girls want stable relationship in the end. If they are in just for lust, then they might go for the stereotypical good looking flirty boys. Some air heads, stereotypical girly girl might as well. But all the girls I have met and talked to, seem to in the end, like a guy who is sensitive, compassionate, caring yet intelligent and smart, hardworking, modest and decent. Such characteristics generally would spell good understanding and a stable relationship for most ladies out there.
So, being an ISTJ, I would say you might fit such criteria. If such is the case, then many girls would like you for your sensibleness (if that's a word).
I don't know what your original question was. I also never seem to know if someone has a crush on me. Its afterwards that people tell me and when I look shocked, they ask- Haven't you noticed? Funny, how despite all these irrelevant details that I observe due to my Si function, I manage to miss such improtant things. Perhaps its because I am not that good at understanding other people's aspirations.