I'd say the skybox is better than your work tbh, not sure if the light-rays in the back are a part of the skybox or if you added them in (
Which looks cool if you did).
(1) The skybox is so bright (
With a really high bloom effect I might add). Your terrain's overall dark lighting doesn't match the lighting of the sky.
(1a) If your skybox is what I think it is, about from the halfway mark, down (
It becomes a solid/blurred grey. The skybox was not finished or completed). You will need to surround the whole area with mountains (
I prefer the "static object>ptest>dirt mound") or something block the player's vision of the grayish blob I see in the far back.
(2) Biggest problem with this picture is that your not rotating/scaling your environment props, for example those trees are the exact same size and rotation and it looks horrible! Same goes for the 4+ bushes I see dotted around the ground.
(2a) Don't be scared to spam the grass a lot, they are very low poly. Change their heights up (
Not randomly, give a method to your madness. Slowly ramping them up to long grass isn't a bad idea) to give some variation to the grass.
(3) Unless your really good at outside environments and know exactly what your going for. I would concentrate on making a single area or point looking really snazzy. They are the easiest to do imo because your all of your work is one area. If you can get one area to get people to go "woooooowww" then they will think better of your entire area, as long as it didn't look like crap.