Right now, your piece feels like collage made from scraps of different colored wrapping paper. I see a very well defined edge where the grass stops and the mountains begin, and the colors and volumes don't really compliment each other in a way that brings everything together.
Imagine for a second that you're looking at your piece of art in real life. Would you be able to see that horizon line so easily? Would the mountains really leave no sort of shadow on the grass? Would there be such a stark contrast between the color of the mountains and the color of the grass? Probably not. The edit I made tried to hold more true actuality.
If you look at some pictures of grasslands or even go outside and look for yourself, you'll notice that seeing each and every blade of grass is pretty much impossible. There are, however, little dips and tufts and bumps in the landscape that you do notice. Drawing grass relies heavily on finding those little abnormalities, and exploiting them to make your flat green rectangle come to life. Those little dips and tufts are generally where you should draw a few actual grass blades. For the rest, you can just generalize, and try to capture the much more important illusion of depth.
The same principle goes for how you shade your tree. Don't worry so much about each leaf on your tree. At least when you're still working on the lighting and feel of the piece. The feeling always comes first because that's one of the most important parts of drawing. Before working on each blade of grass, each leaf on the tree, and the fancy dithering to blend the colors of your sky, ask yourself, "Does what I'm drawing feel convincing?" If it doesn't, then take a step back, and try to find out what it is that's holding it back. I can assure you that it's never the shape of the leaf, or any sort of detail like that. Those can help make something already convincing just a little bit more convincing, but it can't change the core feeling of your piece.
Try squinting really hard on your piece to get an idea of what I'm talking about. Blur out all of the details and see what your piece looks like.
Also, don't be discouraged if your attempt at getting that feeling right flops the first time, second time, or whatever time. Let me show you my first try at editing your piece.
That looks like crap, and it's completely removed from what I think you were trying to accomplish. That failure took me a step closer to what I actually wanted to accomplish though, and sometimes, that's all it takes to get on the right track.
Colors also have a very strong effect on the mood and feeling of your piece. That's one of the problems I ran into with the first draft of my edit. It completely disregarded the softness and calm that kind of defines your piece right now. The main thing I tried to address in my edit was the lack of unity. Every part of your picture felt like a completely different entity, and that didn't feel right to me. In your piece, I see mountains, a tree, grass, and the sun; I tried to blur those lines so that all that was left was a landscape. One single very deliberate idea, with one single very deliberate palette. Colors leave a huge impact on the overall effect of any piece. Here's your piece with a slight tweak in the colors. I didn't touch anything else.
You can keep the mute blues for the mountains if you'd like to keep them really far away, but there will need to be some sort of median connecting the two colors. It could be a
gradual change as the grass and mountains converge, or it could be some other element, like
trees creating a sort of hedge between the grass and mountains.
Now I'd better go before I'm late for work! I hope that this will help.
