- Please don't make us scroll past one million pixels of solid colour. Trim those images
- I don't think the trees benefit from dark outlines. You can use some toned down outlines. The main issue with the trees right now is that they look flat. The light direction, I guess, is just sunlight from above, so you won't have any light from the sides to show the cylindrical shapes. But if everything deep in the forest is darker, then the sides of the tree trunk should be darker, as they're further away. My only advice is to be consistent. If you only outline certain objects, certain trees, certain rocks, then those will stand out as significant to the player. If they're not supposed to be significant, then it will get confusing. (Not talking about very distant background objects, like mountains and clouds, of course)
- Maybe you could show the roots a bit better. If they're connected to the grassy surface, then you could put some grass between the roots, where there is more soil, so the grass looks more organic and not just like a flat carpet.
- It looks like we can see the forest floor in the background, but since this is a side perspective, that wouldn't happen unless there's a hill in the background. And if it is a hill, then I think the tree roots need to reflect that better, by extending further down vertically.
- It almost never looks good when you're mixing different pixel scales. This piece is no exception. Your pixels are 1x1, 2x2 and even 4x4 some places. Render everything with the same scale.
- In order to make the objects in the foreground pop and stand out from the background, you need to rely on either high contrast (e.g. bright stuff against dark background, or dark stuff on bright), saturation levels (e.g. strong colours on a washed-out background) or contrasting complementary colours (e.g. everything in the background is different tones of green, while important objects in the foreground are different tones of red). I guess that's the rule of thumb you're asking for, unless I'm very much mistaken.