Interesting question and interesting responses. To me, it's partly a factor of pulling you closer to the painting. Not just a peevish, peering "gee, how did he do that transition" but "gee, I want to be next to that, I want to touch that." So in addition to values and color there's a paint quality factor - a wall presence - that distinguishes a painting from a photo. I guess for me personally this is why I don't like overly-rendered paintings where every surface has the same degree of polish.
And let's not discount the sheer joy of something that is knock your socks off beautiful. That's pretty subjective, though.
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