I am not a fan of the skull guy. It looks like you tried to create more complex shapes then you should have for his face and in doing so lost a lot of depth. When dealing with this scale the main thing to focus on is depth and lighting (the exercise Adam Atomic suggested sounds very good for that). On the point of lighting, your bird man (who I like a fair bit) isn't lit as well as he/she/it should be. You seemed to have ditched the light direction when highlighting his arm closest to us. By touching up his knees a tad he gains a little more of a supported look and reaffirms a better sense of balance. you can also ditch some colours (probably more then in my edit) and if you want him to have a gut add another highlight in there (didn't do in my edit). Also something to note is when shading your line art you aren't dealing with a colouring book, feel free to break lines up if it will look better.
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Well when you really think about it, it should be much more fun (easier and make more sense) then going over, and over the piece until you work out every single little mistake. Seeing as you have to amend so much regarding anatomy and pose you will have to remove all the details either way as you attempt to fix areas and reposition things. The only difference is doing it the way you are doing now requires you to go over it one piece at a time, which can get confusing and leads to a worse result then I'm suggesting. If your piece had a few small errors then it would be stupid going right down to the basics, but this isn't the case. At present it is actually quite hard to tell what you want the piece to look like which makes it very hard for us to critique. If you go back to a silhouette we can say how the different structures will look and should be positioned, after we say that you have the shape right you can add all the detail you want. By all means keep the colours from the old one, they aren't my primary concern I really am concerned for the anatomy and pose of the piece. Also I don't see why you would delete the old version, is your computer that low on hard drive space.

Really if you can't find something to draw (which is very odd) do some cliche studies. Do anatomy studies, face studies, animal studies, etc. If you keep doing these then you will get good at drawing/art in general. If you look the good pixel artists on this site don't just be really, really good at pixel art techniques, they are good at art in general; forms, balance, volume, colour theory, etc. So no matter what medium you use these are the things you need to learn. I recommend drawing, because you can do it all the frikin time!!! It is quick, you can be sketchy and fast and whirl out a heap of different starts of studies in a couple hours, hence you will get better faster then most other mediums. But in all honesty it doesn't matter what medium you use as long as you do something, if the mediums you have tried aren't 'fun' enough for you then maybe you are an art enthusiast not an artist

I usually have less of an art block problem and more of a stay the hell focused on one piece problem, as I tend to want to jump around so I can't sympathize with you there.

I got Zbrush
It is one sexy program!
First piece decided to go for a face study, not that happy with it as it isn't as generic a face I would have liked. I just found the file which I had thought was saved wrong . Experimenting with new programs is always fun.

My second piece another face study about an hour for this one

This was my 3rd experimentation with the program it would have been about an hour for the low res then maybe 30mins for the high res details (Zbrush has a brush system with alphas making 3d texturing=supa sxc). Just as I finished the low res I found out that the brushes not only have a add function but a subtract function. I knew it was in there somewhere but couldn't find it and it REALLY helps. Faster working for me now.

I think the eyes are really nasty looking also making the main character (I assume from your previous work) human look the most incorrect of all of them.

I made some design edits and some more minor technical edits. Firstly I changed the characters eye, I also messed around with the hair colour and added another skin highlight whilst not adding any colour to the colour count. I personally find the pony tail a little ugly with the scruffy hair design. In your old design however with the more flat hair I didn't mind it as much. I really didn't get the far left characters teeth, I just changed it because I felt like it. I added another highlight to the girl and purple left guys hair to match the main character. I really like the Morrolites design, your edit really changes the character a lot I must say. Prior it gave a very nice Asian elder feel now it gives a very Greek elder feel, either is good (I like a lot  ) but both will have drastic ramification's on the environments that they should be placed in. I changed his lil' hairs, I did this because out of all the characters he is the only one with a light colour outside his lines, A dark extension on his outline is more acceptable and may allow him to better suit his environments. I didn't touch the Ghron. The Chimbies I really unnecessarily used his white as an extra highlight as I felt it looked a little wasted. I hope some of my minor changes and ranting helps a little, though my content seems a little limited in regards to useful information sorry.