I've added more movement to the lion's mane and some hat movement to the scarecrow.  I also added some more dark tones to the scarecrow to try and up the contrast, but I don't know exactly how I would increase overall contrast in the sprites to make them stand out on a background.  Maybe I'll try to make a quick environment in the future.

Adding the Cowardly Lion and fixed the tinman's axe head scaling (thanks for catching that thedaemon).  If anyone sees any issues with the current revs, C&C are welcome.  On to the Scarecrow!!   

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.   

Xion: Thanks for the kind words.

I've added a little more front/back positioning with the shoulders and made the arms swing another pixel out when they come to the front.  I also cleaned up some stuff on the apron so it hopefully looks more like it is flowing with her movements.  I also added a blink to break up the face (she looks kinda like a robot to me).  But if anyone has any suggetions for improving her face, I'm all ears.

As for the slippers, they are emerald slippers in the book.  You'd be surprised how the movie took liberties (most of which, I imagine, were to take advantage of the fancy new (at the time) technicolor technology) even though the books are very clear about how color represents things.

<- OLD    NEW->

I added 3 more colors (don't know if you had intended it to have 16 max) and tried to illustrate where you would get the benefit of some cast shadows.  I also shaved pixels off the boots in some places to give them more shape and to define the heel/toe of the feet.  There are a few colors that can be consolidated overall (although I didn't do that in the edit).  You might try starting with 3 shades of each color (a light, medium and dark tone) to help keep things simple when you start.

Nice work so far.