The biggest area of depth in the Cardinal system is their rotation. They have a strong starting 5, along with plenty of depth to cover any struggles or injuries.
Wainwright is the ace of the staff and one of the premier pitchers in baseball; it's amazing he's never won a Cy Young. The major concern is his usage; the Cardinals need to monitor his pitch count over multiple starts, as he starts to wear down after consecutive 100+ pitch count games (say more than 5 in a row). He's proved that he's completely recovered from 2011 Tommy John surgery. He's off to a great start in 2014; 7 innings, 3 hits, 4 walks, 9 strikeouts in his opening day start.
Miller is hoping to put the questions of last October out of his memory; the young pitcher didn't pitch during the World Series due to shoulder fatigue, which had people questioning Cardinals management. While he might not be the true #2 of the team, he's going to be listed there because of potential and experience; his rookie season was a true bright spot in 2013. His first start of the season will be Friday.
Wacha burst on the scene in 2013, less than a year after being drafted. The pitcher was much better than scouts expected, and further showed why it was a good decision to let Albert Pujols walk (Wacha was taken with the Angels pick in 2012, which was compensation for losing Pujols). The 2013 NLCS MVP should be considered more of a #3 starter until he pitches more than 52 innings; he could regress as more teams see him pitch.
Lynn is an interesting pitcher. He's got the most wins over the last 2 seasons, but fans cringe every time he pitches; Lynn can look unhittable, but often has an inning or 2 where he struggles. Due to this, he might be a better option as a late inning reliever. The Cardinals will continue to use him in the rotation, as he's ready every 5th day and hasn't gotten hurt. There are worse pitchers for the back end of a rotation.
Kelly has been scrutinized this spring for "winning" the 5th starter spot over Carlos Martinez; Martinez did pitch better, but Kelly didn't pitch badly and was a key member of the rotation in 2013. Kelly ended numerous losing streaks last August and kept the Cardinals in the playoff hunt. Kelly should continue to pitch well this season and is a strong 5th starter.