My past comments that I can remember are all about visual storytelling. Not just the art, not just the writing, but how the art carries the story, conveys just the right mood, imparts just the right impact.
The latest one like that was left on this episode of Alone. I didn't spell it out there because I was pretty sure the creator knew it, but the use of colors accentuates the mood/storytelling. It isn't just eyecandy.
I rarely comment on the art alone because that kind of feels like a low hanging fruit. XD; yes it is pretty and my eyes are happy duuur.
Some of the other things I may comment on: expressions, body languages, memorably worded quotes that I strongly agree with.