Yeah, if I'm honest, the foreground tiles are flat and untextured.
Take a look at something - for a random example, Emerald Hill Zone. Look at the way the tiles have been textured and shaded:
And then look at the way the ones you have are:
It does look rather more flat. And then look at the shading on your Metal Sonic sprites and it becomes apparent that they're more detailed.
There we are, I think. Then again, I may just be speaking bollocks.