When it comes to my own somewhat pessimistic projection of 84-78, I think I am being influenced by two things as much as anyone else is: the 'bottom line' effect and the 'recency' effect. Both of those effects fix on the indisputable facts that we were 69-93 last year and that we haven't made any huge upgrades on paper.
Getting more specific about key players, I don't think there's any getting past our two best starters, Lester and Buchholz. Last year they had ERA's of 4.82 and 4.56 respectively. If you stick with the bottom line and the recency, you can't assume that they will be better than that in 2013. Sure, Lester pitched better than that the last 2 months, and Buchholz had a great stretch of games in mid-season. But no amount of parsing makes the 4.82 or the 4.56 go away.
I'll say this: if both Lester and Buch can at least give us a combined 390 innings and each knock at least a run off their ERA's, the Red Sox should make the playoffs.