On the surface this might look like an easy question to answer, but, it is one of the most difficult questions I've come across recently. The problem I have with trying to answer this question is that it takes more than one thing to get you to say I do. Looks, personality, intelligence, frugality, forethought, empathy, or modesty to name a few are not enough if they are singled out to make you say I do. On the other hand, if the woman in question would possess a combination of these qualities then you will quickly say I do.
The same would apply when it comes to getting you to walk away, although for some people a single bad habit or problem may be enough. I've known people who have walked away because the lady they were interested in was a smoker. If you are just starting your relationship it easier to walk away at the first sign of something you don't like, However, if yours is a relationship in which you are emotionally invested in, it will take a combination of things before you leave them.