A note before we begin: I believe minor league stats matter, and I believe minor league stats, and particularly AAA stats, are very valuable tools. See here for a good primer on Minor League Equivalencies, which Bill James showed closely approximate major league statistics. I also believe that what you did over the last month generally matters a lot less than what you did over the past two or three years.
Let's begin by comparing the Indianapolis statistics of two players:
Andrew McCutchen: .303/.361/.493
Garrett Jones: .307/.348/.502
These are very similar numbers, and yet my attitudes toward these three players throughout the season were very different. I loved McCutchen, but was actively hostile to Jones.
Neither of these lines look particularly good in translation. For example, if you adjust Jones' AAA numbers to Pittsburgh, you get .259/.295/.410, which is pathetic. (Remember, that's not a prediction of what he'd do in Pittsburgh, it's a translation of what he actually did.)
Of course, translating minor league numbers to the majors often makes players look pathetic. The reason they're in the minor leagues in the first place, at least in theory, is that they aren't good enough to play in the majors yet, so it's only natural that translating their numbers would make them look bad.
The reasons why I clamored for McCutchen and not for Jones, then, are twofold. First, McCutchen is 22 and Jones is 28. Players generally improve until they are about 27 or so, and then they start to go downhill. So there was good reason to think that McCutchen might actually outplay his equivalency as he improved throughout the year (which is, in fact, what happened); there was no reason to think Jones would do so, at least not judging from his numbers. Also, McCutchen is an outstanding defensive player, whereas Jones has no real defensive value. McCutchen can post a .750 OPS and still be valuable, whereas Jones can't.
Obviously, Jones, who has hit .301/.366/.607 for the Pirates so far, has exceeded any reasonable expectation. But why did it happen? It's not like the equivalency was "wrong"; remember, the equivalency is a translation, not a prediction, and the existence of outliers like Jones doesn't disprove it. He's hit for a higher OPS in the majors than he ever did at any minor league level, which is really strange given how old Jones is. Possible explanations:
1) Genuine improvement. At this point, this explanation deserves some attention, since 19 homers in 239 at bats are very hard to explain away.
2) "He learned it on the plane." I'm copping the title from a message board poster many years ago who was trying to understand why Tike Redman hit so well after being promoted in 2003. There was nothing in Redman's minor league history to suggest that he really was the sparkplug he briefly seemed to be in the majors, so the opinion of this poster was that if Redman had learned anything that caused his great half season in 2003, he must have learned it on the plane from Class AAA Nashville. Of course, Redman hadn't learned anything on the plane, which was exactly the point--his 2003 performance was a huge fluke. A complete fluke isn't as likely for Jones as it was for Redman, whose excellent half-season was driven mostly by batting average. But it's certainly worthy of consideration. For example, Jones' major league equivalency suggests he would have been a .259 hitter if his stint in Indianapolis had actually been in Pittsburgh. If he'd hit .259 in Pittsburgh but generally retained his power, that would make a lot more sense to me given his minor league profile.
At this point, the smart money has to be on a blend of "genuine improvement" and "he learned it on the plane." This isn't so bad for Jones, but I do think the Pirates should probably pencil him into the lineup next year rather than writing that name in ink. Besides, even if his hitting for the Pirates this year has mostly been legitimate, players who develop late often have very short peaks.
We should also be wary of Neil Walker, who has parlayed one great month at AAA in two years there into a big league callup. Walker's profile, going back years and including his terrible .311 OBP at Indianapolis this year, suggests he has no idea how to control the strike zone, and big league pitchers will have little trouble exploiting him. He does have legitimate power, with 47 extra base hits at Indy, but not so much that he's worth the effort. He currently compares unfavorably to players like Tony Batista or Pedro Feliz, who both play good third base defense and have excellent power but frustrate their teams with .250 batting averages and .290 OBPs. Walker currently has less power than either of them, so he should stay in the minors for now. The fact that Andy LaRoche isn't exactly tearing the cover off the ball is neither here nor there; he's very probably a better player than Walker right now, and Walker needs to prove, at the very least, that his terrific August represented real improvement rather than simply one great month.
In the second half of the year, Indianapolis' lineup also featured Jose Tabata (who did what he does, hitting for average and little else) and Jeff Clement, who tailed off badly after a strong start after coming to the Bucs in the Jack Wilson deal.
Indianapolis' best pitcher was... well, actually, it was Tom Gorzelanny, who had already exhausted the Pirates' patience and is gone now. After that, it was Daniel McCutchen. No Relation pitched well enough to overcome some rather glaring issues with flyballs, and earned a spot in the Pirates' rotation, where his flyball issues quickly reared their ugly head--he's allowed three homers in his first two starts, although he's pitched pretty well otherwise. Elsewhere, Brad Lincoln took some time to adjust to AAA after a great start at Altoona, and he finished his season with two great AAA starts. Virgil Vasquez, meanwhile, pitched like the decent AAA pitcher he is. The best pitcher out of the bullpen was Chris Bootcheck, who mowed down International League hitters before running into a wall in the majors.