I'm of the opinion that most playoff series odds are no better than about 55-45, even when one team is clearly superior to the other, because of the randomness inherent in short series and in baseball in general. The playoffs aren't about finding out who is the "best" team, but determining a World Series winner. And while sometimes those two coincide, often they don't -- which is a big part of why the MLB playoffs are more interesting than the NBA playoffs, in which the best team nearly always comes out on top.
Nate Silver and Dayn Perry wrote a story for Baseball Between the Numbers that showed the small but significant effect a few team factors had on playoff success: three good starters, a good closer and good team defense.