Twenty years ago Bill James made the somewhat startling claim that minor league results, "properly evaluated", predicted major league results as well as major league results do. He then made some statistical calculations called "major league equivalents" that purported to translate a player's minor league results into equivalent major league results.
Looking at James original calculations, there are people who were never successful in the major leagues who had outstanding "minor league equivalent" seasons. There are now several different systems out there to "properly evaluate" minor league players using statistics by creating their major league equivalents. It is difficult to completely evaluate these system sbecause most of them are moving targets that get "refined" each year. (A problem that is ubiquitous in the sabr community.) Perhaps as important, it is not clear what the criteria are for success. Most of the recent systems don't even attempt to evaluate players below AA and James himself made no claims about pitching results.
The problem is that sometimes major league success predicts future major league success. And sometimes it doesn't. The same is true of minor league success. In fact, when "major league equivalences" are criticized for their failure to accurately predict major league success, the usual defense is to point to similar failures in major league statistics. There is no doubt that statistics are crude tools for player evaluation, but for identifying prospects they are even cruder. There are many career minor league players who have had success while only getting, at most, a sip of the big leagues.
So why is minor league success not an accurate predictor of major league success? The answer is really that minor league baseball is not the same game. This is not to suggest the rules are different. Or that the skills that make a successful major league player won't make that player successful in the minor leagues. But some of the flaws that exist in every minor league player, the things that keep them in the minor leagues, allow other players to be successful even with their own flaws that keep them in the minor leagues.
We have all heard about the young hitting prospect who fails because they never learn to hit a breaking ball. In the minor leagues, there are a lot of pitchers who can't get their breaking pitches over the plate. Which means the hitter can have success by laying off the breaking ball and only swinging at fastballs. Most major league pitchers will just feed them a steady diet of breaking balls in the strike zone unless they show they can hit them.
Likewise, there are minor league players who have to "cheat" on 90+ fastball. Fortunately, they can cheat in many situations since they know a fastball is coming. Its the only pitch the pitcher can throw consistently for a strike. And the pitchers who can control their other pitches often lack 90+ fastballs. If they had stuff and control, they would already be in the major leagues. The flip side of that, for the pitcher, is that with a 95+ fastball, you don't need to throw anything else because, even cheating, that hitter is going to have a tough time catching up to it.
So what are some of the other differences?
In the minor leagues hitters will chase breaking pitches out of the strike zone, while major league hitters will lay off it and force the pitcher to throw strikes.
In the minor leagues, a pitcher can have success throwing hard and harder. In the major leagues, they need to be able to change speeds because no matter how hard they throw the hitters can catch up with it if they know a fastball is coming.
In the minor leagues, you can be a successful hitter with large holes in your swing because many pitchers lack the control within the strike zone to consistently take advantage of those holes. Major league pitchers largely don't.
In the minor leagues, you can have success sitting on fastballs. In the major leagues, you have to be able to hit a pitcher's breaking ball.
In the minor leagues, a pitcher needs to be able to throw the ball over the plate. In the big leagues, the pitcher needs to be able to throw quality strikes and locate their pitches in the strike zone.
Of course major league players have flaws too. Which brings us to the most important difference. There is a huge jump from the minors to the majors. People tend to think of baseball talent as a pyramid with the only a few players at the peak. But the reality is that it is more like a mountain, with the slope becoming steeper and steeper the higher you go. The difference between AAA players is not remotely comparable to the difference between big league players.
Just consider the difference between a major league star who gets 33 hits in 100 at bats and the marginal major league player who gets 25. A comparable difference for the next level would be a hitter only able to get 17 hits, well below the Mendoza line. While the jump from the minors to the majors is large, its not that large.
Does that make minor league results meaningless? No. But it means that without understanding the context of those results you can get fooled into thinking a minor league player is better than he is.
How about worse? Are there players who struggle in the minor leagues and then blossom at the major league level? Not really very often. Where they do, the issue is usually less their skills than that the minor leagues focus on player development. Players are sometimes asked to do things that reduce their immediate success in order to develop the skills they need to be successful in the big leagues. Pitchers, in particular, may be working on pitches that they wouldn't be throwing if success were the only criteria. They are working on a changeup or locating their breaking pitchers. And managers will leave a pitcher in the game even when they might be struggling. Likewise hitters, especially as they move to a new level, may be asked to shorten their swing or change their approach in other ways that reduce their immediate success.
The key thing to remember is that it is a different game when most pitchers throw 90+ and most hitters can hit a 90+ fastball. Its a different game when most pitchers can throw breaking balls for strikes and most hitters can hit a breaking ball. Its a different game when most pitchers can locate a pitch in the strike zone and hitters don't easily chase pitches that are out of the strike zone. Major league baseball is a different game than minor league baseball. And while minor league success is not meaningless, its not always meaningful either.