I'd say it's mainly colors and composition. What I notice is that you only use skin color on your characters, while the skin has in fact countless of colors, reflecting the surrounding light. This goes for backgrounds as well. They'll appear more lively when you widen the range of colors you use. As for composition... you tend to fall back a bit on the basic poses en portrait. Try something different every now and then ^^
Let me start off by saying you've got tons of potential. Really c: So here are some thing I think you can improve on:
1. Contrast: I noticed in some of your pieces (namely your original character) that the difference between your highlights and shading is a little nominal. Darken those shadows and brighten those highlights and your piece will pop more. 2. Line Weight: This is more for your drawing pieces, less for the painty ones. Line weight is kinda something you have to feel for every individual piece (or maybe I just don't know how to explain it, I dunno), but something you can start with is making the lines of the drawing in 1, thinner brush and then outlining the entire outside of the character with a thicker line. It's a good starting point for understanding how lines work to make pieces bolder or softer. 3. Anatomy: Your understanding of anatomy is okay, but you'd be surprised how much you can improve by doing just a few figure studies or sketches. If you have the money and time, try to get in a figure drawing class. If you can't, maybe ask a friend or family member to pose for you while you sketch them. If you don't wanna do that, there's plenty of stock photos floating around the internet that you can reference. And remember, these sketches don't have to be entirely accurate or detailed, and you can take as much time as you want with them. Try doing just a few quick skeletons of people for fun. 4. Try Different Mediums: From the looks of it (I could be wrong), you're just using Photoshop. PS is okay, but other tools like Sai are much friendlier for digital painting. Same with line art. Again, PS is okay, but Illustrator is the industry standard for nice, clean line art. And when you work on a single piece, don't limit yourself to any one program; you can easily do a sketch in PS and bring it over to Sai for painting.
Anyway, I hope that helped a little. I'm by no means an expert, but I figured I'd at least try to give you some wise words from my own experiences c:
Honestly this is great, and I appreciate the compliment. I just recently got into coloring my work too, so I'm still getting the hang of contrasting the colors so they pop a bit.
It's definitely going to take a bit more time. As far as the anatomy is concerned, you're exactly right, sketches of the body are the way to go so I can up my knowledge on how the body looks in different poses and at different angles.
I've never heard of Sai or Illustrator, I guess I'll check out their websites and see what they're all about.
Have you studied the absolute basics of art? Like, drawing spheres and pyramids, color scales and human anatomy until you 'get' it? Basically, do you have a good, 3-dimensional understanding of what you are trying to draw, rather than copying from eye or memory?
Such exercises are damn great to throw into your regular drawings. Things tend to get a lot less stiff that way - spend a couple of minutes each day just sketching basic things you see, and there will be some improvement - the more you try, the better you will get, as with everything else.