Unfortunately there's a lot of noise on the tree, especially the leaves. They don't "read" (Look or appear) like leaves.
In regards to the background i'm not sure why it's lighter at the top of the hill? I could be wrong but surely it should darker?
The clouds could do with being more "round". You should look at how other pixel-artists have dealt with clouds and how they portray them.
But hope that helps! I'm not good at pixel-art but those are a few bits of advice/critique i can offer.
Just remember to keep trying and don't automatically assume because you've finished a piece you've actually "finished" it. Sometimes you can go back to it and see mistakes or see things you could do better, which leads to totally re-doing it or just touching it up, there's always something more to learn!
