The K rate. Addressing craig's point, I'm less concerned about Hartshorn because he's a few years younger than Mathis and sports a much lower strikeout rate, which tells me he's selective, but has excellent pitch recognition skills.
Mathis has a great swinging strike rate, so it's not like he's hacking away at bad pitches, which is why I said I'm in wait and see mode with him. However, if he's consistently letting hittable pitches go, particularly for called strike 3, it makes me think there might (*might*) be some gaps in his game that could be exposed at higher levels.