There are of course multiple factors so I'll start with the most obvious one, physical.
A pretty face is very, very attractive to me. However, so long as she isn't ugly, she has my attention. That is the most important attribute to me of the physical, everything else is secondary. I prefer longer haired women with fair(ish) skin. I do prefer curvier women with a nice bust and ass, but it's not at all required. So long as she ain't a skeleton and she isn't going to crush me, I'm fine with her body. I prefer overweight to underweight, as weird as it sounds.
Her personality and so forth are by far the most important factors for me, I'd much rather have an average/ugly girl that I can talk to and engage with than a beautiful one that bores me. On that note, she needs to be relatively smart. I don't mind if she's smarter than me or dumber than me, so long as she's got some basic smarts. I'd prefer that she is opinionated but open minded. That is, she can form and has her own opinions on things but if we disagree she will be willing to change her mind, as I am willing to change mine. [Depending of course on the outcome of the discussion/debate and of course whether we agree significantly enough]. She needs to be sweet, I'm fine with taking flak when your emotional but if you are angry or upset all the time without rhyme or reason, I'm not interested. I always treat people with respect and act as nicely as I can towards them, I expect the same back and that's especially true of someone with whom I am thinking of dating. There are others, but I'm getting bored and no-one really wants to hear this and it's getting pretty long as it is.
Suffice to say, if she's moderately attractive and a nice enough/decent enough person, I'm interested. I'm not really interested in non-committal relationships so they have to be pretty bloody attractive for me to consider it, for me most of the attraction with the other gender has always lain in getting to know them and being with them.
[EDIT: On a side note, I sound so bloody egotistical and up-myself here. I don't like it but it's 7:00am and I haven't slept yet, so I can't be stuffed changing it. This is simply what I find most attractive and interesting.]