The majority of his improvements have come on defense at shortstop. He's been a +3 DRS (Defensive Runs Saved) defender this season compared to -9 DRS last season at shortstop alone, adding another -7 DRS at third base. He essentially cost the Red Sox a win and a half in 2014 simply with poor defense. Because shortstop has been such a less-than-stellar position overall recently, however, Bogaerts managed to still be worth about 0.4 WAR despite having a batting line of about 18 percent below league average.
So is Bogaerts finally about to become an above-average starting shortstop? He has been so far. But there are warning signs. His batting line of .302/.337/.412, while excellent and about 6 percent above league average, is being propped up by a .350 BABIP (batting average on balls in play). His career low walk rate of 4.8% isn't encouraging but his 14.8 K% is a huge improvement compared to his 23.2% last season. He's using his speed to much greater effect, with 16 infield hits on the season after having that number in all of 2014. So a higher BABIP is probably sustainable., although perhaps not at this level
Another thing to look at is his pitch values. The sinker and slider devastated Bogaerts last year. He posted -3.81 and -2.57 runs/100 pitches against sinkers and sliders, respectively. This season, those numbers are -.032 and -0.05. He's making better quality of contact by being more aggressive.
If Bogaerts keeps up the defensive improvements and can continue to bat at around a league average clip, Boston may truly have their shortstop of the future. He may never be the perennial All-Star they were hoping for, but right now, he probably is the best shortstop in the American League. With all that's gone wrong with the 2015 Red Sox, Bogaerts has been strong and steady.