Well to answer that I think I'll have to try to explain what I perceive to be retro pixel art.  There are a lot of examples that would work just as well but I'll pick a screenshot from snake's Owlboy:



To me, this is about as retro as it gets.  Not timeline-wise, as we have pixel art that references C64 or Atari or art, but in terms of intention or goal.  Owlboy seems to be trying to elicit from the viewer the reaction of "hey this looks just like SNES!" or perhaps more accurately "this looks just like SNES but better!"  And most pixel artists out there, it seems to me, are going for the same reaction, even if they aren't actually making a game and are just making sprites or mock ups.  That "I wish I could play this" reaction, which seems to aim specifically as though these pieces of pixel art were something you could have popped into your SNES at one point.  These styles deliberately appeal to the viewers (and the artist's) nostalgia, which is, I think, incredible indulgent.  Which is fine; I can't deny that I look for a download button when I see a screenshot like this.  I don't know snake's story but I imagine it goes like this: he got into pixel art because he wanted to make games like he used to play when he was young; over the years he got incredibly good, even better than most of the old 'masters' and still makes art like he wanted to see in a game when he was young.  That may not actually be snake's story, but it's a common enough story regardless (it would be my own story if I had ever gotten good at pixel art).  Most of the old talented pixelation members are still making big-headed sprites or muscly warriors or the like as though there is nothing else to do with the medium, and that's what most people want to see.  To me, this sort of continuing progress in technical skill is, while impressive on one level, ultimately boring, because most of the art is still focused around the sensibilities of games from the 90s.  In my view not much better than the 'bad pixel art' stuff Gil was talking about that focuses completely on the nostalgia trip.



(from DESIGN REBOOT HD)

But superbrothers... it is retro, to an extent, and I wont say it isn't indulgent at all--I think you'd be hard pressed to find any art that isn't indulgent in one way or another--but it seems much more self conscious.  The left screen looks a lot like a videogame, and superficially looks like it is all retro--it's lo-fi, you can see all the pixels in a relatively simple style, and it is some sort of warrior we like we could see in an old-school RPG.  But I can't imagine that existing when I was a kid.  It's anti-retro because it doesn't appeal to nostalgia like Owlboy does because nobody made games that looked like that back then--and I don't think many people would have liked to see it back then either--most people wanted a prettier Zelda or a more badass Contra or an upgraded game of choice.  It eschews old game sensibilities like the bright colors or big muscles and heads or dramatic poses.  I guess I see it as anti-retro because the big pixels make me want to say "just like old games" but when I look at how it's presented I just think, no, games weren't like that at all.  



I had a lot more trouble articulating that than I thought I would... I'll continue to think on the subject.