The Red Sox made a ghastly mistake (in Theo's absence btw) when they traded for Josh Beckett. Sure, they won in '07, but they didn't HAVE to win in '07 (unlike '03-04) -- and they crippled the team for the future. Beckett + Lowell + Lugo was $30 million a year, and Dice-K, with the posting fee, added another $20 million or close to it. And even Beckett's just not that good -- in fact, in the 50 years I've been following the Sox, he's easily the most overrated player they've had. Think of it this way: His career upside is that he'll have been as good as Derek Lowe, who's never been anybody's #1 starter.
That's not even mentioning Hanley Ramirez and Carlos Pena, who should have been half of the best IF in baseball. The Sox can't go out and get a hitter as good as Ramirez (proof: he's a lot better than the guy they couldn't afford las year). Instead, they're stuck with a bunch of mistake hitters (Lowell, Bay, Ortiz now) and over-selective guys (Youk, Drew). Good pitchers make few mistakes, and through the strikes the selective hitters take.
Having a whole bunch of above-average players is just not the same thing as having Manny, Papi, and Pedro in their primes. The expected result is exactly what we saw this year.