Aren't you asking the wrong question? You ask what's wrong with him and not how good is he really? The truth is that he's not much better than Jim Negrych. He has more bat speed, a better arm and runs better. He's also 4 years younger. And even with that, he's not much better. Hit skill players become quite good at beating minor league pitchers. They become minor league all-stars. They win minor league games. Dig deeper and something like the speed you speak of becomes an example of what's wrong.
He runs well but he's not really that fast. He's quick and that's fine but then you look at his body type and how it projects and realize that he's expected to fill out like Eric Hinske did. Across the board you get a player who is pretty good at a lot of things but only great at hitting line drives off of minor league pitchers. There's no elite bat speed. There's no elite power. There's no indication that he can adjust well to MLB sharp and late break with his swing.
I'm not saying that he won't succeed or that he won't be good. I'm saying that without one projectable elite skill, it's hard to become a star. What's the difference between Pillar and Kirk Nieuwenhuis? End of the day, that's a pretty fair question.