You've pixelled the hair as if it were flat to his head, it need more volume, you'll notice the hair in the picture is almost as tall as his face.
You've also done what is called "pillow-shading", meaning instead of having a definite light source, you've just made the outsides dark and gotten lighter towards the center of the sprite. You need to decide where your light source is, then add highlights and shadows accordingly.
The pose is boring and static, as well as unnatural. No one stands with their feet that close together. Decide what you want to do with his spine, because straight vertical is never exciting. Get rid of the "Lego man" pose by widening his stance and doing something with his hands, rather than having his arms down by his sides. The interior black lines aren't doing you any favors either.
You've got far too many colors, and this is because you've got colors that are way too close in value to the neighboring color. Add more contrast, makes the light colors lighter and the darks darker.