For a reliever, Santos' repertoire is tough to match. He has one of the best fastball-slider combos in the game, as well as a tailing changeup that can fool hitters when he's feeling it. I think he lost faith in it last year, as he didn't seem to use it as much as the season progressed, but he can get by on two pitches alone.
The biggest physical question about Santos is durability -- not injury, just workload. He's pitched three full seasons in his life, and he's pitched 42 (when including Arizona Fall League), 52 and 63 innings. In both his pro seasons, he started to fade soon after breaking into new innings territory. To give you an idea of what he's like when he's fresh, though, he's never allowed a run in April.
He also seems like he's subject to negative momentum more than your average closer. This could be perhaps due to refining his feel, but he also seems like a high-strung guy in general, which might make him more susceptible to streaks and slumps. He talks a mile a minute (sorry, 1.6 kilometers a minute), and when he starts missing the strike zone, you can see him start to get a little worked up. A.J. Pierzynski learned to use the mound visit more than he normally does in such situations, just to slow him down.
Basically, he has some of the best one-inning stuff in the league, but he does require a little more management than your typical shutdown closer. That said, he could very well earn that status in a year or two.