The Twins starting pitching is not going to dominate many games. So they are going to have to deliver on offense. Like most teams, the middle of the lineup drives the offensive numbers. The Twins have three guys that can match up pretty well with any team in baseball when they stay healthy. Mauer, Willingham and Morneau need to put up big numbers for the Twins to be competitive.
As the Twins number three hitter, Mauer lacked the home run power you want in that spot last year. Ideally, Mauer would regain the power he had a few years ago and hasn't shown since. But at a minimum he needs to be among the league leaders in batting average and on base percentage with some extra base hits
Willingham supplanted Morneau as the cleanup hitter last year. That put a right handed bat between Mauer and Morneau. Opposing manager werer forced to choose between using his lefty specialist against Willingham, or letting Mauer hit against a right hander. Willingham doesn't necessarily need to repeat his career year, but he needs to come close.
Morneau slowly recovered from his concussion last year. His July and August numbers looked a lot closer the Morneau of old, hitting over .300 on a n annual pace of 20+ home runs. The Twins need him to repeat those July and August performances over a full season.
Simply put, the Twins cannot afford to lose many close games in the ninth inning and stay in the race. That makes Glenn Perkins almost as important as three players above.
Glenn Perkins took over as closer mid-season last year. Twins handed him a contract this winter based on his taking over the closer role. They need for him to succeed in that role over a full season. If he doesn't, we will likely see the rest of the team pressing to provide big leads. That is a formula for cascading failure.