It means good bone structure. It has nothing to do with attractiveness. If you are an artist drawing portraits, you want your subject to have well defined features.

Here, "defined" means sharp, clear, easy to see. Whether or not that is pretty is a matter of taste or perhaps fashion. It's not obviously positive or negative, but rather noncommittal.

It is a description. It is a compliment, because nobody would say it if they didn't like it. Imagine an artist making a line drawing of a face with a pencil. They can't use shading to suggest contour and curvature. They will be drawing only well-defined facial features. Defined facial features give a sense of individuality and personality. Just as an illustration of meaning, in these photographs, the person on the left has well-defined facial features. The person on the right, less so.

Yes, “defined facial features” is usually a compliment. It means some part of your face stands out as memorable (You could have pretty eyes, a nice nose, pretty smile… etc. etc.)