What's impressed me so much about Shelby Miller this year and Michael Wacha on Thursday is just how... unfussy they are to watch. This is a symptom of something they have in common, and not the commonality itself. It's not entirely "real," either; most pitchers will look cool and efficient with a batting average on balls in play of .263.
But I think it might be a lack of traits that are immediately impressive that ties them together. Carlos Martinez and Trevor Rosenthal and Joe Kelly, and before them even guys like Eduardo Sanchez and Maikel Cleto, took on almost cartoonish proportions in their call-ups. They threw unusually hard (or in Sanchez's case, with unusual movement), and even when they struggled it was clear they had some thing that would be effective against major league hitters.
Which, weirdly, made it much easier to demote them after they were called up. It was simple to watch Eduardo Sanchez; say, "Hey, champ, that's a killer slider!!"; pat him thoughtfully on the head; and send him back to AAA to "work on things," "things" being all the undifferentiated stuff that we can't easily separate into a nasty pitch or one gaudy stat.
Neither Michael Wacha nor Shelby Miller has one pitch that will make you pause MLB.tv and wave your friends over, but it seems like they're basically done Working on Things.
There's a lot behind the apprehension that's greeted the Michael Wacha call-up, but I think that's part of it. Carlos Martinez's prospect luster has almost nothing to do with how close he is to dominating major leaguers; it's tied up in the idea that he will dominate them eventually, and one recognizably impressive pitch is enough of a taste for now. Michael Wacha's prospect luster comes from how ready he's looked, and how quickly, and once he's called up he either looks ready or he doesn't.
One start in, Michael Wacha looks ready. And he'd better, because he's not the kind of guy who gets patted on the head.