1) its massive bibliography (102 pages), which is supplemented by the bibliographies provided at the end of his discussion of each passage;
2) its extensive introduction dealing with historical, literary, and theological issues (Harris gives an able defense of the letter's unity and interestingly argues that Paul's "opponents" are a combination of i) proto-Gnostics; ii) Palestinian Judaizers)
3) its readability (Harris certainly doesn't avoid technical issues - consider the series, after all - but I got the sense that he was aiming to write a very "reader-oriented" commentary)
4) its lengthy discussion (45 pages!) of 2 Corinthians 5:1-10 (Harris essentially suggests Paul is here arguing that believers who die before the parousia - Harris argues that v. 6-10 do not refer to the parousia - are in the immediate presence of God but are nonetheless in some sort of disembodied state; cf. J. Cooper, Body, Soul, and Life Everlasting)
5) its extended paraphrase that helps clarify the exegetical decisions outlined in the commentary proper (those who opposed dynamic equivalence translations would certainly disagree but I would love to see more publishers include extended paraphrases in future exegetical commentaries) .
Undoubtedly, a proper NT library would need to include more than this single volume on 2 Corinthians. At the same time, one could not have a proper NT library without it!