TINC pretty much said everything that needs to be said.

So I'll just say: Listen to him. And welcome to PJ.

Welcome to pixeljoint : )
I didn't mean to write a novel, but here goes:
    The lines thickness and jaggedness indicate you haven't really paid attention to the individual pixel placement, which is an important aspect of pixel art (in b4 opinionated counter-argument).  The size is also rather large for a pixel art piece, especially one with such little detail.  It's generally best (and easiest, as you have to have control on the pixel level) to work as small as the canvas can be without loosing any quality of important details.  As such, I'd recommend scaling this down quite a bit (perhaps even 25% of the original size shown here), and then refining the line work and polishing it up.  As far as the piece itself goes, there is relatively little contrast between the shades used on the skin, so I'd up the contrast in order to not waste colors (color conservation is generally an important aspect of pixel art- though not a requirement) and also to make the highlights actually noticeable.  I also find it odd that the shirt has shadows but no highlights, whereas the skin has highlights but no shadows.  As for you concerns about coloring, you may want to try hue shifting between shades, as value is not the only important aspect of a palette.

EDIT:  also, I noticed the text on the preview isn't made by hand, which is a no-go on this site.
also, if you work from a reference image, it's best to post the reference used.

Hope I was helpful, and let me know if you have any questions.