Flats
Flats & Shade
Flats & Shade & Highlights
Flats & Shade & Highlights & Subsurface Scattering
Flats & Shade & Highlights & Subsurface Scattering & Blush
Sorry for the long post but I wanted to break down I’ve gleaned from others. Plus with the pictures set up the way they are you can use your keyboard to cycle through them to see the differences.
Out of all the steps in the above process, I think shading makes the biggest impact. Having great line art can really help but the first bit of shading pushes the object to have a 3d appearance.
In giving the character life, it’s a toss up between the subsurface scattering and blush. They both do a good job pulling their weight. Even with my blue guy. 
Cycle through the images and post comment on what you think is the best addition to the flats.