DDP's Sprite Workshop

[kidicarus29]

Great job!
If you care for criticism, the bricks in the background don't stand out enough; I'd pick a light color for them, but not the same color as the background. It definitely does well portraying the fact that they aren't solid, but it doesn't easily portray the fact that they're in the foreground. For example, the ground(background) in the pit seems to be in front of the background brick blocks, yet has a shadow cast over it. It looks very strange. It also looks very inconsistent, because, at some points in the scenery, these brick blocks seem to be implied to be on the same layer as the main ground(it's in front of trees, bushes and other background elements); in others, the design implies it is a background element(it appears behind floating blocks and solid ground).

Mario's right(our left) leg seems to need its "outline"(red line between his legs) moved to the right. As it is now, that leg doesn't quite look like it's the one closest to us. By the way, I would add something in that blank space in the middle of the pipe. It gives off a shape similar to a cube with diagonal edges(if that makes any sense). Koopa's shell looks strange; as if his arm is protruding from the sides of his shell and his shell actually continues on for a bit past his shoulder. This is a problem shared with both versions. The Mystery Blocks and Brick Blocks both look strange without any outline on the bottom. The bottom row of grass on the grass tile should be moved up a bit in my opinion. It looks pretty strange as is. The clouds and bushes look like orbs, though this may be(and probably is) intentional.
Also, this is just a suggestion, take this however you want: for the jump-through grass tiles, I'd slim the tile down to the top(the NES can recolor 8x8 sections of a graphic) to portray more so that the tile isn't solid on its sides.
I feel like Mario's other shoulder should be slightly showing in this screenshot; just a little bit. It may not be possible to do without making some major changes, though. Goomba's eyebrows should be thicker(if you're going for an official look; if this is stylized, disregard this). Goomba's mouth seems to continue too far to its right(our left), it throws off the angled perspective.

I agree with the majority of what you said, but I do have a few comments to make.

I think the bricks are actually in the background, not the foreground. This is because the Koopa is over the bricks on the right, meaning they must be behind him. If not, he would be inside of the bricks, and I don't think that's right.

Mario's shoulder could be showing but, you're right, it would require some major changes in order to look right, I believe.

Something could be done about the shading on the pipe and the blocks such as the mystery ones, but I'm not entirely sure what.

And yeah, the Goomba's eyebrows should definitely be thicker.