It's generally recognized that baseball teams play better at home than they do on the road. One reason for this is the way the game is set up; the home team always gets a chance to respond to any runs scored by the opposing team in the ninth inning, while if the home team breaks a tie in the bottom of the ninth or in extras, the game simply ends.
The other major reason, however, especially in the modern game, is that when teams put their rosters together, they focus on players who will play best at home whenever possible. A player's performance at home is by far the most important of any environment, since his team will play fully half of its games in that ballpark.
That brings us to the 2013 AL playoffs, the Boston Red Sox, and home-field advantage. Thanks to the American League's 3-0 victory over the National League in the All-Star Game in July, the AL will enjoy home-field advantage in the World Series, which means that the top seed in the American League will enjoy home-field advantage throughout the postseason. This year, that's the 97-65 Boston Red Sox, and they will enjoy significant advantages over any lineup that comes to visit them, posting a team OPS of .819 at home and winning 53 of the 82 games they played at home, a .654 winning percentage.
Fenway is a bizarre park, which is part of its charm -- the abbreviated left field with the giant Green Monster, the short wall in right field, the disappearance of foul territory in the outfield 10 to 15 feet past the end of the infield dirt -- which makes it less welcoming to teams not calibrated to take advantage of its uniqueness. More importantly, it's harder to defend for visiting fielders, particularly outfielders and especially left fielders, who are unfamiliar with its dimensions.
There is, in fact, only one team that has hit better at home than the Red Sox this year: the Detroit Tigers, who will play the Oakland Athletics in the divisional round. The difference between the two teams (the Tigers had a team OPS of .821 at home, compared with the Sox' .819) can be attributed to the influence of Miguel Cabrera (1.081 OPS at home, 1.074 away) on the sample, though he spent most of September looking nothing like the player he was the first five months of the season. If he's not back into full form for the postseason -- and considering his intermittent injuries this season, he may not be -- it's more than fair to give the Sox the nod in this department. The Sox have a better staff ERA at home than the Tigers do, but the Tigers have a better staff, the best one in the AL playoffs, though it's not without weaknesses. Both the Sox and Tigers have concerns in their bullpen, something the teams they're facing, the Rays and Athletics, rate as comparative strengths.
All of that said, the Red Sox sit prettier than any other team in the AL at the beginning of postseason play. Not only are they arguably the best home team in baseball, with home-field advantage through the postseason, they're also the best hitting road team in baseball, with a .773 road team OPS, far above the .714 league average.
A high home-team OPS can mean that the team is well put together to play in their home park, like the Red Sox in Fenway, or that the run environment there is already through the roof, like the Rockies in Coors Field. What does a high road OPS mean? Generally, it just means the team is good at hitting baseballs and scoring runs. There's no normalized run environment -- there are more plate appearances against divisional rivals than against teams from other divisions or leagues, but no one division is so tilted in favor of hitter's or pitcher's parks for this to matter that much. (The AL and NL Wests each feature one of the most hitter-friendly parks in the game, Coors and the Ballpark at Arlington, and one of the most pitcher-friendly parks, Petco Park and Safeco Field.) One expects road OPS to be depressed a bit, because teams will be putting their pitching staffs together with an eye towards playing better as home as well, but when a team is hitting 60 points higher than the league average line for the full season, it's a pretty good indication they're just very good at their jobs at the plate.
So if the Red Sox have been that great both at home and on the road, especially at the plate, who can stop them? The short, easy answer is, just about anyone still in the playoffs. Things like "best home team in baseball" or "best road hitting team in baseball" aren't guarantees, but probability factors. It's something that tilts the field in Boston's favor, but it won't pitch or hit or play defense for them, and in a short series -- even in a seven-game series -- season splits tend not to mean much of anything, even if they come in relatively large samples.
By that same token, it's foolish just to ignore splits; they are expressions of how certain situations regularly turned out over the course of six months, and they can tell us certain things. For instance, they can tell us that while the Tigers are the most likely team to face the Red Sox should Boston advance past Tampa Bay, the Oakland Athletics could be just as dangerous a matchup, if not moreso. Oakland sits second in baseball in road OPS behind Boston with a team-wide .760, which is actually substantially better than the .730 they posted at home -- that makes intuitive sense, because we know that O.co Coliseum is a pitcher's park (even in with a .730 OPS, the Athletics still hit relatively well in their home park -- the overall line for all hitters at O.co this year came to a .695 OPS; compare that to Fenway, which played host to a full season of .752 OPS ball from all hitters). The strength of this Boston club is their hitters; if they can neutralize them on the road in their cavernous home park in Oakland while hanging with the Red Sox offense in Fenway well enough to take a game, the Athletics could win a series against Boston even with the Sox playing good ball. The Rays on the other hand, who don't have the offense that the Athletics have shown this year, would probably need a bit of help from great individual performances and lucky bounces to work their way by the Sox, and the Tigers' chances against just about everyone depend on which Miguel Cabrera shows up: the slow singles hitter from September, or the league-wide terror from everything before that.
With all the chips stacked in their favor and the fact that Boston's a legitimately good team this year on top of that, if this AL postseason was played through 10 or 20 times, I think it would be reasonable to expect the majority of the times would end with the Red Sox representing the American League in the World Series, but in the real world that's still very much an "if," not a "when." Home-field advantage is a known quantity, and there are few greater enemies of the known quantity than playoff baseball.