First of all, I like the ball...even with no mouth

And yeah, I think what you pointed out there is what he means by not fitting. It does look a bit...anti-gravityish[?] in my opinion...I think it would look better if it looked more "solid" to the ground.
Also, I think there are some problems with the tree. The big one is where the leaves meet the trunk, the trunk overlaps the leaves and is square. Also the repeated pattern that the tree is composed of isn't very appealing, an neither is that mushroom[?]. It needs to be more defined as a mushroom.
I like the smiley face in the rock tile, nice detail, but there needs to be more contrast in it. The shadows of the specific rocks should be more like...on the right side of the right picture O_o, I don't know if that's the emulators fault, but it looks better.