Before the book, I thought Piper was critical of Wright's view of the final justification because he thought Wright was advocating a view of “merit-based” salvation by which our good works earns God's final verdict. But this isn't the case for Piper clearly states that this is not his problem with Wright’s view. Piper, reciting Wright's understanding of the final judgment, states:
"In other words, Paul believes that all men will face a final judgment (law-court) in which people will "be vindicated, resurrected, shown to be the covenant people"- this is, justified by works. When he says "by works," he does not mean by legalism or by merit or by earning, but by the obedience of our lives that is produced by the Holy Spirit through" (pg.104)
So, what is Piper's problem with Wright's view of the final judgment? I believe Piper doesn't want good works to have a direct connection (basis) with our final vindication, which he believes will prevent the" double tragedy" of not glorifying Christ properly and hindering assurance of salvation (pg.186 -187) . For Piper sees good works as having an indirect connection with our final vindication, that is he sees "the necessity of obedience not as any part of the basis of our justification, but strictly as the evidence and confirmation of our faith in Christ whose blood and righteousness is the sole basis of our justification" (pg. 110). A diagram of Piper's view would look like this:
WORKS --->(evidences of being in Christ) --->CHRIST--->(basis of final justification)--->FINAL JUSTIFICATION
And Piper believes that Wright seems to be advocating a view of good works that have a direct connection (basis) for our final vindication (pg. 125-128). A diagram of Wright's view would look like this:
WORKS---> FINAL JUSTIFICATION
(I know this linear diagram above is too simplistic, and it doesn't take into account other elements of Wright's thought so to do some justice to him, it's important to note 3 things that relate to his view of the final justification: 1) Christians are connected to Christ's death and resurrection which provide forgiveness of sins and new life. 2) Christians are given the Holy Spirit to change their hearts so that we can have faith and good works 3) A Christian's future justification is based on the pattern of one's life is certain because of present justification by faith (Assurance of Salvation)) .
Again, it's important to note that Piper problem with Wright isn't that obedience is "earning" our final vindication (pg.104), but his direct connection between good works and the final justification- in Piper's view good works are only evidence that we are in Christ, who God looks at for our final justification.
My personally thoughts on this matter is that both these options are acceptable. For no one is saying that our works are meriting or earning our salvation and both seem to believe in 1) unconditional election 2) substitutional atonement and 3) perseverance of the saints.
But what about the direct connection with good works and the final justification? This issue must ultimately be addressed exegetically, but for now I take some comfort, knowing that conservatives like Simon Gathercole, who believes in imputation, appears to see a direct connection between goods works and the final vindication. And it also seems that Piper's indirect view of works just pushes forward the process of God's examining our good works, which in his case, is to see if we are in Christ which then allows us to be justified on the basis of him rather than Wright's view of works directly effecting our final justification (Again, the issue involved isn't related to merit).
Finally, what about imputation? This is a difficult issue and once again I receive more comfort from Gathercole, who states:
"A statement by Robert Gundry on the (non) imputation of Christ's righteousness in particular has sparked a response by John Piper, and Gundry and Don Carson have also entered the same debate from different stances. It is not my purpose here to enter this debate. But it should be said that there is clearly a great deal of diversity of opinion on the matter. This is, of course, not sufficient in itself to let discretion take the better part of valor. But in case, the diversity seems to arise out of the complexity of the New Testament evidence, not because one side is particularly hidebound to tradition and the other wallowing in the desire for novelty or for doctrine that is more amendable to culture. I would not myself deny this traditional understanding of imputation. Still, because of the complexity of the issue, I would propose that the requirement that is is specifically Christ's righteousness that is imputed to believers should not feature on evangelical statement of faith. To make such a finely balanced point an article of faith seems a dangerous strategy. Nonetheless, it is very clear that justification is still Christological through and through. Both the cross and the present action of Christ are the vital grounds of justification."
 See my pervious post on my thoughts on Piper's "double tragedy"
 See Wright's "The Shape of Justification" and "Justification, "The Bibical Basis and it's Relevance for Contemporay Evangelicalism".
 pg 223 of "Justification in Perspective: Historical Development and Contemporary Challenges", edited by Bruce McCormack