@Crunchino Good for you for wanting really thorough critique. Most people don't want it and can't handle it, but you're right, it helps you grow faster. I hope that more people than me take you up on this and give you good feedback. I'm pretty good but I'm still learning too, there's probably going to be a bunch of stuff I'll miss.
I see you've used a lot of mirroring. In some ways that's fine, since it'll cut down on how long each part takes to create, but you need to go back over it once it's mirrored and touch it up so the mirroring isn't obvious. It's the most obvious at the seams where you're getting whole rows of double headed blades of grass and thicker than average shadows.
The cliff walls have a very obvious dark line running through them and there are a few places where they don't match up when tiled differently than the norm. It'd be good to add a few more brighter spots to the dark line and break it up, plus add a little of the darker spots to the main section of tiling. It'd be also good to have a little more variety in size with the clumps but I'm not sure how much work you want to put in on this.
The trees are looking very plasticy right now. Part of this is the high contrast of the colors, part of it is the high saturation in those colors, and part of it is the pillow shading.
Here's my favorite tutorial for explaining pillow shading. It's not a lot of text but the pictures make things very clear. I think once you fix the pillow shading the colors will look less jarring, but it's something to consider tweaking at some point. Right now the trees look ripped from a different game.
The hollow log looks out of perspective. The opening on the one end is possible from that point of view, but if it was really like that the inside would be shaded very differently. I think the easiest way to solve it would be to put the log at an angle and add a bit of cast shadow.
I think with how you've got everything else colored it would be good to up the contrast a little on the metal on the treasure chests. It's not very shiny right now.
A lot of the problems I'm seeing are difficulties with the way tiles work. Do you have a good method of checking how things tile as you draw? I'm a really big fan of Pyxel Edit for this, since the only other way I know to do this is by mapping and checking and fixing and reimporting and checking and fixing, ect. It's rather tedious. I'm fairly certain there are other art programs that let you map tiles and draw on them (ProMotion?), but I like how Pyxel Edit does it best and it's pretty cheap.
Overall I'd say you're doing a good job. The colors are rich, the textures are interesting without being overbearing. there's a good balance of high to low detailing and overall it's pleasant to look at.