The first thing that sticks out, or rather doesn't stick out, is how the upper levels blend with the lower. I think the quickest fix for this would be some shadows, but it's not necessarily the most effective fix...you may need to look into it more.
Next, I'd like to say that the "grid" is apparent. Luckily, the tiles look like they could easily be fixable...some copying and pasting could do the trick.
Lastly, you say you want your dirt to look more "dirty", but it seems as though you are already on the right track; just continue what you've got going with texture-y dirt tile and you should be fine.
I can see these tiles going places, and with enough work, they could be phenomenal. Keep pixelling!