Hello holden, I don`t really know what to say because the concept and composition are awesome!!! that is the real valuable thing!!!
In terms of a matte painting execution I think you are in the correct path, I have to say is a very difficult concept to make it a matte painting, some parts are like very sketchy, maybe you can make the border strokes softer, also the light is overexposed or very bright in the right part, may be yo can try with an option call render (filter-render-lens flair). Also I have the feeling that the hole image is very grainy or sharpened, but man 5 stars, a lot of concept information in this!!!
- Drop the lensflare
- Fix the perspective issues on lower-left castle
- Break the symmetry of these rock-formations on the middle island path / if it is some kind of hand-made structure, make it look more like it / if it is dragon skeleton, then the skull is facing wrong direction.. anyway I have problems seeing what you meant it to be.
I might be a bit late to crit, but the thing that disturbs me is that it doesn't have a depth of field and also is overdone with glow. You should work out a way to establish some kind of atmospheric perspective to add reality. Try adding some fore-mid-background elemnts to it, they should help.
I think that if you might use a more darker approach that it would add to the ominous feel of the island it's self. Think a dark and hellish picture like that of Pirates of the Carribean, where fire and fog lurks in every corner of the island. At the top of the island is a rock, carved from the very bone of the legendary master dragon. Having such light and glow in this composition kind of takes the punch out of the mystery. I love the concept, its strong and leads the eye around the picture well.