Brutal? Alright,
So the sunset thing is already covered, good.
The clouds, to start - if you look outside you don't see a single cloud every minute go flying by like it's late for something. Clouds drift by lazily, and rarely do you see small single clouds sparsely dotting the sky. Look into cloud formations and aim for something more realistic. I'd also ditch the hard shading and black outlines, clouds are wispy, fluffy things.
Grass can be tricky, especially when working with vector graphics. I'd check out Adam Phillips work for reference, the Brackenwood series. Hard shading can be more effective here, and add texture if possible. The hill itself is very round, an uncommon sight in nature.. change that up for sure.
The tree isn't bad, but try to incorporate shading to add to the realism. While you've avoided the typical thick trunk with the knothole and fluffy perfect green leaves, it could still use work. Trees are never really that color either, look outside and see. Look up trees on Google and grab colors from pics you find. Add swatches you create to your color palette for repeated use.
Other points..
- Add depth by bringing in foreground or background elements, it'd make a huge difference.
- Sun is really big, scale that down maybe. Keep the soft gradient edge though.
- Add other subtle details such as leaves twirling in the wind, bird high up in the sky
- Consider texture, everything isn't flat. Texture = shadowing. Consider your light sources too.
- Reference the real thing to avoid first grader cartoonish style.
