"Maybe if the lighting were like..., then it would enhance..." That's a real good point, but still on the subjective side. However, it's an opinion, and well within the guidelines, in the spirit of "helpful...." I see something with a glaring error which detracts from the image, like a body part poking through clothing, or a non-seamless texture tiling obviously, that probably ought to be mentioned. I guess you could also try to second guess what the guy had in mind but didn't quite achieve. In any event, I'm far more likely to offer a suggestion IF the artist specifically requests it. But I think that should be up to the artist, and not de facto implied in the Comment field.