@Katherine Hi. In my opinion, your art and style is great. It has personality and it is not going to be liked by everybody (don't have to), but sure it's going to find its public. Me, personally, I love it! It reminds me German Expressionist cinema (The Cabinet of Dr. Caligari for example) with a pinch of Tim Burton.
Some people are going to feel that it does not fit children's book's style. I completely disagree. They said the same for "Where the Wild Things Are". Children are fascinated by a good dark atmosphere.
So my advice is, continue to do the same thing.
But now I'm confused. You say "I desperately want to know everything that is separating this illustration from professional quality art that could get published in a picture book"
Let's get clear, professional not always means "good quality" in the business sense. But it means "good quality" in the art sense. And it is not a black or white thing, there's a lot of gray area in between the middle of the illustrations that are clearly in the bad quality area or clearly in the good quality area. For me, your illustration is clearly in the good quality area, but that doesn't mean that you can't improve. You always can improve, always.
I say I'm confused because you clearly know all this. I mean, I think any person that can achieve this level of craftsmanship has to know, more or less, how good his work is, and has a sense of direction.
The question here is, do you want to do your own thing or do you want to turn more commercial style to fit the mainstream market?
I think you have your own style and I would go in that direction. Working everyday, making one piece after another, building a body of work, consolidating and improving your style.
This way you'll perhaps don't find a place in the mainstream current, but you'll find your niche. And if the mainstream current goes your way and finds you, is in your terms.
To have your own style is a big thing. Some people spend their entire life searching for it. You have it and my advise it is "let it roll" "be yourself" "explore that"
Now if you want, for commercial purposes or otherwise, to fit the mainstream market, I would go and read all the award winning books of the last 5 years and then every book that sells good. Study what they are doing and see how you can change your style to fit the market.
About personal critiques, be careful. There's a difference between quality and style. In my opinion you always have to critique quality concern issues and not style. I mean, the critique always has to mean to improve the illustration and not change the style of the illustration. A lot of people critiques, go in that direction, they try to change the illustration to fit his vision or "the rules" they learned. The rules has to be learned and unlearned, only after you unlearn them do you know you have it. People who are in the learning state usually are slaves to the rules and their work looks forced and they give sometimes terrible advice because they don't recognize a good expressive work or style, they only look for the rules to be taken care of.
So, for me, your work is fascinating and doesn't need changes, only evolution. I was attracted when I was a child to this kind of style and I am now. I don't think making the child look more like a child is going to improve your work and, on the contrary, it can take away its personality and atmosphere.
But it is your call. Have a great day!
I know you meant your comment about there being a sombre note to my artwork as a critique, given that I'm angling towards picture books, but actually that describes all of my favourite art so if I'm hitting that note, I'm thrilled!