I really, really wanted to love this piece, because it has a ton of features that I adore in it, but something about it just doesn't gel with me.
I love limited color pallettes, like what you have here. The extra splash of pink in the mouth and ear were an excellent decision. Your dragon is wonderfully detailed. I love how rough it looks, it looks like a scary monster. Those teeth look dangerous. I also really like the way you did the sky outside, with the rising moon-looking bit to prevent the image from being too dark overall. Lots of truly good things are happening here.
The cave walls... I like less. They look like hairy tree roots a bit.
However, the biggest problem with this piece, in my opinion, is that the dragon does not stand out compared to the background. When you isolate the dragon, the dragon is wonderful, but when put in context the dragon looks awkward and like a mashup of the cave walls and the sky. If the white portion has been colored in, it would have been infinitely worse. Having the very top of his head completely black like the sky when it's directly touching the sky makes it less distinct, and makes all of the shading on the dragon look superfluous. It took me second to realize it was a dragon creature and not just a really weird transition from the sky to a cave wall.
Honestly, I held my thumb over the upper-right cave portion and the picture looked much more appealing to me, even with his head being completely black on top. He just looked more like the focus of the image.
I'm not sure how I feel about the eye. Every time I look back on it I flip-flop on whether or not I like it. I will say that it feels reasonably appropriate for a mythical beast.
Despite my nitpicks, this really is a well-done piece. It shows off your talents well.