This is just great. It makes me really happy to see all of this art. You're doing a great job, and I'm impressed with how you stick with it. I really like seeing how it has improved over time. The character art especially gets better and better.
I'd like to offer some critique, as I feel like I understand some of what jackson31 was getting at. Something I feel, especially in the darker scenes, is that it lacks interesting variety for the eye, giving it a sort of 'dull' feeling. On top of this, the darkness is making your textures less saturated. To combat this, lights can be used, and perhaps you can imply the darkness while introducing a little more light.
A nice example of variety of lighting is Trine. Take a look at some screenshots. Many of those environments look dark, but they make excuses to light them in creative ways, so it's never dull.
Also a great reference, especially for your game, is Wildstar. Check out this picture.
http://www.wildstar-online.com/media/uploads/images/media-screenshot-images/wdstr_screen_025.jpgLots of color, and it still feels good while giving a nice dark underground impression.
Another shot from Wildstar.
http://www.wildstar-online.com/media/uploads/images/media-screenshot-images/algoroc21.jpgThis one has some interesting light colored blue fog giving the depth a bit of a glow, as well as breaking up the darkness with interesting glowing pieces.
So my suggestions would be to use more variety of light, even in the darkness, and maybe if you can, try to imply darkness through use of textures, and some colored lights or fog to give the feel of darkness, without losing so much color and variety.
I don't mean to get down on your work, as I really enjoy it. Just a small critique