For the jab, I would recommend not having his feet shift like that. A better idea might be having him do a quick step forward, or leaning a bit more into the punch. It looks pretty nice and clean, but it has almost no range. Leaning forward or a step in might help that.
For the kick, I'm not sure if this makes sense, but move the 'weight' of the leg's pixels to the top. Right now, the leg looks a bit rubbery and an inverted bone structure.
I also wouldn't worry too much about shading for right now unless you want to keep the character like it is now. I would assume you plan on adding clothes and details that would have different shading than bare skin.