One of the biggest things I can see is that there's not much contrast in the sprites, and the shuttle and side containers seem to be pillow shaded.

Your outlines should be well below the middle shade, and you may benefit from using less colors and maybe relying less on solid and consistent outlines.



Notice how on the side containers, the shading seems to be moved over one pixel, so the outer outline is lighter than the one near the middle. Also, the middle orange part only uses four colors which gives it more contrast a less plasticy look. Now, I made this in about thirty seconds, so it's not perfect and I didn't edit the ship, but it should help.