In general, it looks like everything is very busy (visually) and it all sort of blends together. The sprites look nice out of context, but they just don't read when composed in the scene. You might want to consider less saturation or brightness for things that should "recede" visually and add brightness, contrast and/or saturation for things that "advance" visually. Also, consider making the ground tiles less busy by making them have larger shapes (all the rocks, dirt and grass blades are visually similar in size).
Things like the bricks just draw you eye too much because they have a lot of contrast and a very tiny, repeating pattern. You could try "implying" bricks rather than just drawing each brick individually. By this I mean only drawing a corner of a select few bricks here and there. Or just add a small highlight to a couple bricks (not every one).
The trees and bushes need more brightness and contrast to make them stand out over the ground. You might also consider making the ground or trees a slightly different color, so that it all isn't a similar tone of green. Maybe define a shadow color that you can work into all tiles.
Even though they aren't the tiles you made, I thnk the most successful tiles right now are the path of cobblestones. They read very clearly and your eyes are drawn to them (which is what you would hope a path would do in a game).
Overall, I think these tiles have a lot of promise. I often find I have to work and rework my color choices once I actually put the tiles together, so I don't think it's an easy process. You just get better with practice. Hope this helps.
