What separates the disappointment of the 2012 Red Sox (69 wins) and the first-place 2013 Red Sox (94.5 win pace)?
It's hard to point to any one thing; their pitching has improved, as has the offense and defense. The improvement in the offense is, I think, the most interesting because it hasn't been a result of the superstars stepping up, necessarily:
D. Ortiz 2012 OPS: 1.026 2013 OPS: 0.985
D. Pedroia 2012 OPS: 0.797 2013 OPS: 0.770
Nevertheless, the team has clearly improved to the tune of about a half a run (4.53 runs per game to 5 in 2013). Some of the increase has resulted from better health and the play of Jacoby Ellsbury, however, a surprisingly large amount of the improvement is unaccounted for until you get to Jarrod Saltalamacchia, who's sneakily developed into one of the best offensive catchers in baseball.
'Salty' has long been considered a potential masher behind the plate. He came up as the top prospect in the Braves deep farm system, topping out as the #1 ranked minor leaguer (BA) in 2007 - ahead of Elvis Andrus, Matt Harrison and Yunel Escobar among others. That same year he headlined a trade package for Mark Teixeira. Within the next 12 months he was buried by fellow tradees Neftali Feliz, Harrison and Andrus in Texas and was left in a rotation that left him in AAA more often than not. That said, he didn't exactly dominate the PCL (a notorious hitters league) and spent much of his time on the DL.
Exactly three years later after constituting a large part of the Teixeira deal, Saltalamacchia was dealt to Boston for three decent but not impressive prospects. The Red Sox hoped he could inherit the catcher position from Jason Varitek smoothly, but he ended up falling relatively flat. Despite significant power (41 home runs) he didn't crack a .300 OBP in his first full two seasons as a Red Sock (2011-2012) and by all accounts was an average (at best) backstop defensively. The mediocre catching was/is basically expected, he's a big dude at 6'3" 245 lbs. The offense was basically underwhelming. For example, in 2012, he posted a .222/.288/.454 batting line. Needless to say - less than impressive.
In 2013, though, Salty has made a huge jump and is a big reason for Boston's improvement as a team. In a pretty much full-time role he's been great across the board .272/.341/.456 in near full-time work. Compare that to the average MLB player .261/.327/.414. And that includes all players; consider that Salty is significantly above this mark as a catcher. That's been marked improvement for Boston. And while it doesn't account for the entirety of their improvement it's a significant reason they're sitting atop the AL East.