I think that's a great idea to explore other genres. For me, I would say a portrait has lasting appeal when I no longer view it as a portrait, but as the person, someone real. When the artist has somehow depicted that person, who they are, what they are like and their character. Harder, I think with sculptures, which although 3D, have a barrier somehow, which always makes you think of it as a sculpture and not what is depicted. In fine art, I tend to low key and subtlety rather than the subject first. Depends on what you can live with, I guess. I guess, in summary, the lasting power of an image is perhaps a measure of how well it has resonated with the viewer. Is that a measure of success or that indefinable DM? Could be. Since we are the sum of art we have absorbed, loved and loathed over our lifetimes, it isn't surprising when someone else doesn't 'see' what you see. If I had to quantify it, it has to be lasting appreciation. Being in love with it, however many times you revisit the experience of looking at a piece of work.