ok first of all 15-30 minutes isn't bad at all - especially for a nice looking basic background 
I dislike the blury skies and moon. They should be at least a little bit sharper. The "painted" feel of your work is very nice though and reminds me of works done in "Corel Painter".
To make it look better and more detailed you just have to make brush smaller and smaller and refine everything in iterations
Same thing with doing grass - start with shadowy and highlited big spots on the grass and refine them until they look good (ie. like grass)

I dislike the blury skies and moon. They should be at least a little bit sharper. The "painted" feel of your work is very nice though and reminds me of works done in "Corel Painter".
To make it look better and more detailed you just have to make brush smaller and smaller and refine everything in iterations

Same thing with doing grass - start with shadowy and highlited big spots on the grass and refine them until they look good (ie. like grass)
