Well, my 2 cents:
Your first two, plus the second to the right, thumbnails/compositions look very good. The others are OK, but there’s something lacking… It’s hard to point out exactly what, since I don’t think I could to better myself. However, I think they would need more work on the proportions and look for a more simple statement in terms of shape, light and form. Great compositions, in my opinion, tend to follow the great Colin Chapman’s (engineer and founder of Lotus) device: “Simplify and add lightness”. Look for example at Eytan Zana, whom I think has absolutely great compositions. They look deceptively simple and are so clear in their statements.
As for your rendered piece, I think it has a few problems:
The shapes are too uniform. They’re somewhere between realistic and cartoony.
Lighting needs to be pushed more towards that clear, simple, statement. There’s no real visual interest in the light.
Edge control. Rocks should look like rocks, snow like snow and ice like ice. A lot of your edges are too ambiguous.
Story telling. Looks like you’ve tried but that little, faint, figure isn’t doing anything. It’s more like the typical concept art trope of putting a vague figure in the image. It’s low contrast also makes it look meaningless.
With that said, I think you’re really on the right track. Your work looks pretty decent in my book, and I’m not sure I could do better myself. But, you asked for criticism so at least I can offer my thoughts. Hopefully there are others here more qualified to get into the more technical bits.
By the way, I have myself recently transitioned from a more generalist approach towards doing 2D. That’s something I wish I’d done earlier, since a generalist approach isn’t worth much and I’ve found that I truly love drawing and painting. So I wish you the best going from 3D to 2D.