I think this is a very open question that is based on personal preferences. You're saying one can't mention "fat" however,some men don't think that a woman being overweight is unattractive.Some men want a lot more meat on their girls bones. Gaps between the teeth...i have a big ole gap between my two front teeth that will remain there forever because I don't give a sh*t. Yeah some guys may hate it,but then some guys have told me it's beautiful and to never change it. So it's really a personal preference as to what some like and don't like. But that goes for anyone. Somethings may be rejected and others accepted.
Everything can be improved.. If you have any reason to feel insecure, figure out why YOU think that is (or ask a really hot guy around you, what you could improve to look better) and then take care of each thing as necessary.
When I first started, I thought my smaller frame would be unappealing towards most women - I figured it out after several times in public that my frame is just fine, it was my mindset that was making it unattractive.
its weird I love certain piercings on girls like belly button , small nose stud , tongue . but there is certain weird piercings I hate with a pashion . like weird facial piercings , in noses , lips , eyebrows . its weird I like piercings but certain ones a major turn off . so throw a ugly lip stud on a cute girl and it ruins it for me