Firstly, there are parts of that image, such as the tail of the pony that are pillow shaded. this would be fine if the light source is directly in front of them but that would mean everything else has to be shaded the same way. secondly, either your outlines are too heavy, or you should just use shades to define the limbs and details of the pony. as you can see in the image you like a lot, there is no heavy outlines at all, thus removing a cartoony aspect. Thirdly, your shades are all too close together, don't be afraid to have large changes in shades, it really compliments the lighter colors and helps define your light source a lot better.