A woman's eyes are the main thing that makes me take notice. Beyond that, I prefer them to be a shorter than I am (which isn't all that difficult due to my height), but not so much shorter that I have to get down on my knees to look her in the eye.
When I was younger, it took long blonde hair and green eyes to turn my head. Now, I go for women with kind of an olive complexion and dark hair. I'm not into the long hair that much either. Around collar length, or with some asymmetrical cut/style to it, is best.
As for weight/build, it kind of varies. I don't want to be with someone who looks like they'd break in half when confronted with a strong breeze, but I'm not interested in a girl who really lets her body go. I guess I'd say I prefer a woman who is fairly fit, but that isn't a deal breaker. One girlfriend gained twenty to thirty pounds during our relationship due to medication, and I didn't really see it. Of course, me not seeing it didn't go over well either. I want someone who at least tries. I don't have a perfect body, so it isn't fair to expect one - I just want someone who tries as hard as I do, if that makes sense.
I also tend to appreciate a more natural look. I'm not saying no-makeup, but more along the lines of what she would wear for herself, as opposed to what she would wear to try and look attractive for someone else or to try and stand out in a crowd of others doing the same. There was this one girl I usually only saw at events, or in clubs. She was always made up, and always looked good. I ran into her at the store one weekend. All she'd applied had been eyeliner. She looked amazing to me, and I asked her out on the spot.
I don't really go in for women who are always dressed to the nines. I think I've only dated one girl who wore dresses regularly. A girl in jeans a t-shirt, or a babydoll shirt, gets me every time. Especially if there is a band logo on it someplace, depends on the band of course.
Tattoos don't bother me. Out of the last four girls I dated, three had at least one tattoo. Two of them actually had the same tattoo, which is kind of strange.
It isn't really external, but I also go for women with some connection to music. Singers, drummers, bass players, flautists, voice instructors, etc.
Smoking is a big deal breaker though. If I wanted to lick the bottom of an ashtray or watch someone else I love destroy their entire body, it might be different. Unfortunately, those two items are not on my list of things to do.
The same with drinking. I don't drink, I don't get drunk. I'm not saying a woman has to become a teetotaler, but the second or third time I have to literally carry her because she is too smashed to walk, or if a friend has to point out that the girl is falling over drunk while stripping in the parking lot to show some guy all of her tattoos, that is a pretty good sign I don't need to put any more effort into the relationship.