First, you've got a lot of visual noise. Try to smooth some of it out!
Second, try reworking the dirt so the transition ends closer to the top rather than being as spread out.

This seems like a problem with poor basics, honestly.
Do some art studies (figure drawing especially, there's guides for this on youtube!), try to break the torso and various other parts down into basic shapes rather than just doing stick figures. It'll give you a more solid base to work from when you move on to finals.
There's an entire forum masterpost here about the basics of art and pixel-art especially, check it out here!
http://pixelation.org/index.php?topic=19594.0

I'd say finish the sprite rough before coming for critique, there's stuff I can say here but until I see the hips and legs I'm not sure how valid it would all be.
Her arms seem somewhat long here, for example, but I don't know if that's because the legs are out of shot or just because the shirt terminates earlier than you'd expect, you know?

Actually, I recognized the bat character before I saw you mention it. She reads pretty fine, good work there!
As for color choices, I really don't know! The ones you have right now give everything a vaguely mature look, like faded renaissance paintings. This works well with the semi-realistic style you're rolling with.
The dithering in some places might need some touch ups but for the most part it also adds to that and gives the skin some texture, it's very easy to imagine these scaled up into non-pixel portraits.

I think the biggest problem area is the lines in some places, right now everything here has a slight jaggy look that detracts from the end result a bit. Do another pass over your various curves and see if you can tweak the curves to be more smooth. To point out some of the examples I mean, the rightmost elf girl's hair needs some very slight tweaking along the outline and so does the far-right girl's curly coat fluff!

So can it be said that one thing that makes the left sprite bad is too high of a contrast between the green and the blue?

sort of but not really! Read the responses again, to boil it down to one line the big problem here isn't that the contrast is too high or low.
It's that because the two colors are both vying for attention by nature of being so bright they wind up drowning each other out.

The other part is that, within that green especially, there's not enough contrast between the various parts of the shading, giving it too soft an appearance.

Here, I redid the left sprite with updated colors to show off possible fixes without changing it beyond that, all I did here was darken the green some!


EDIT: And a compiled gif, to better see the changes!