In a rather depressing note, the Red Sox have actually allowed more runs than any other team in the league. The park certainly has something to do with it, but this is hardly a team built on pitching and defense right now. Another contributing factor may be that the Red Sox have faced off against division rivals 24 times so far, and AL East teams average 225 runs scored, versus 175 for the AL Central and 172 for the AL West. If one were rather cynical, you could argue that's because those teams get to face the Red Sox and the Orioles' pitching staffs.
Looking at the Pythagorean won - loss records, using Bill James' quick and dirty formula - none of the teams really jump out as over or under performing. However, I'm not sold on the Blue Jays as a real contender - they've only played 12 games against the AL East, while the other 4 teams have played at least 18 games against divisional opponents. Also, their team OBP is a meager .310; while their power laden lineup offsets that, I don't think the starts by either Vernon Wells (11 HR) or Jose Bautista (12 HR) are sustainable.
Finally, in case there was any doubt in your mind, the Rays are for real. In fact, they're the best team in baseball, and it isn't even close. The Rays are second in the AL in runs scored and first in runs allowed. But that doesn't really show just how strong their pitching has been. Despite all those numbers above about how great the AL East is offensively, the Rays have allowed 128 runs this season, which is 36 runs better than the second place Mariners, who have the luxury of a huge stadium and a weaker division. If there is one weakness with the Rays, it might be their relief pitchers, especially with JP Howell missing the remainder of the season after reinjuring his shoulder. As the season goes on, we may see some of their top tier minor league pitchers filling a relief role, much like David Price did during their 2008 World Series run.