Hey Xaefo. My first thought is that you need to understand the basics of drawing first and foremost. Pixel art seems like it should be easy, but it actually takes a lot of knowledge of anatomy and how things work together in order to get something to read on such a small scale. It took me awhile to realize that it was in fact a rhino head and not an alien with some antenna.
In all art, and especially in arms and legs, you need variation. If they're the same, then it will look static and boring. Honestly, you could have done anything else to the arms and legs and it would look better. Mirror images are rarely a good thing, unless you have a specific purpose behind it.
Also, I'd work on how to draw a head in 3/4 perspective. Unless the entire body is in a profile position (fully side, like Mario), then the head needs to look three dimensional too.
When a character is one color (except for his undies) then you'll probably need to use some shading to break up the flatness of the coloring.
Here's a quick edit to show you what can be done in just a few quick minutes:
One last thing on posing. First start off in all black. Make a silhouette of what your character is doing. If you can tell from the silhouette who a character is and what he is doing then you can start going into colors and details. Here's an example of silhouettes from the game Team Fortress 2. All are very unique and instantly recognizable.
I hope nothing I said was too harsh.
