you need to look at the peripherals, not just the plain stats.
The guy had 25 infield hits last year. He's had an over .400 BABIP in the minors. It's absolutely not sustainable. You normalize his BABIP, and instead of being a .300 hitter, he's a .250 hitter, and he has a sub .800 OPS (since his luck due to poor fielding in the low minors doubly inflates his OPS)
He does have good plate discipline and takes walks, yes. But he's got horrible plate coverage, and strikes out over 25% of the time. He strikes out more than Jay Bruce (who strikes out a lot - but has a ton of power). Maybin struck out more in the minors than Adam Dunn, Ritchie Sexson, BJ Upton, Grady Sizemore - pretty much anyone you can think of who strikes out a ton in the majors AND WAS AS YOUNG AS MAYBIN AT SIMILAR LEVELS. He also grounds out nearly 60% of the time. It's pretty impossible to have a ton of power when you have a 60% ground ball rate and a 10% line drive rate.
Maybin needs to dramatically change his entire approach at the plate. It's not merely becoming a better hitter, he needs to become a completely different hitter.
If you look at his slash stats alone, you are missing the (quite clear) story.