The Cubs had one of the worst pitching staff in baseball last year. They subtracted Z and added Volstead. They are auditioning for two spots in the rotation and three in the pen. I don't see how they are going to move many spots with what they have and that's if Marmol can still throw a slider.
They added Volstad, Maholm, and possibly Wood. That could be over half the rotation.
There are three big reasons the rotation will be quite a bit better than last year.
First is the depth. 49 starts combined last year between pitchers that people knew were not major league caliber starters and proved that last year: Russell, Lopez, Ortiz, Coleman, and Davis. This year, unless you think Samardzija will crash and burn miserably, the number of starts from pitchers like that should be under 10. That's a whole season's worth of starts from one starter plus a little bit.
Second is that of the pitchers remaining, based on past history and peripherals Dempster and Wells have more room to go up than down while only Garza has more room to go down than up. Maholm and Volstad don't have huge hurdles to cross to be equal or better than Z and Wells were last year. That leaves Samardzija/Wood to replace all the horrible starters the Cubs threw out last year.
Third is the defense. The Cubs made two defensive upgrades (3B and RF) with only one downgrade (1B). They have two upgrades that will likely come up midseason (CF, 1B). Byrd should continue to decline defensively in CF, while Castro will likely improve at SS. Overall, the defense should be better, especially in the second half of the year. That will help the rotation, especially the pitchers without huge strikeout rates.
Could the bullpen be bad? Absolutely. It should be better when the starting pitching goes deeper in games which will test the middle relief less, but they also have downside risk especially if Samardzija moves to the rotation. That one is a lot harder to project than the rotation though because of the few innings and the rotating cast that will likely be in the bullpen this year.