Experiment

Hi. ;D I ended up here from your comment on PK's speed tutorial. And I think the technique you were going for is well done! Has a very paint-y feel to it. :3

The only thing you're kind of missing is any sort of light consistency. The light seems to come from the back (on the pony's body) and at the same time from the front (on the face). Even if I assume its coming from high up on the left side, the shadows on the body/hair just don't work. You said you wanted the pic to pop out more, shadows help for that.

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^like so.
(I arbitrarily chose one direction for the light here. Probably not the one you were going for(?), just trying to illustrate what I mean. I wanted to not change it much and stick to your style.
Now the light all comes from the back, which makes a strong light on one side and - because the light reaches around the pony- a faint glow on the edge of the other side. Hair now has more of a drop shadow and the mouth got a shadow too. Point is, all light from one direction.)

/is totally not an authority or anything, just thought you might find that helpful?