I love both of John Piper's and N.T. Wright's writings and I also believe both of these guys are precious gifts to the church. Beyond this, I know Piper's upcoming book will do justice to Wright's view of justification even though they will probably and ultimately disagree because of their different views on "imputation of Christ's active righteousness".
I believe Piper's biggest problem with Wright's view of justification is his belief that works will some how "merit", "earn", or "be credited for" our final justification. In some sense, Piper is right to note this, for Wright clearly states that Christian good works will some how be "credited" for our final justification. Wright states
"What we are not encouraged to do is to draw up a checklist of things done and not done, to weigh them against one another and thereby to arrive at the final verdict. This suggests that Paul is being careful not to endorse the merit-measuring schemes that, despite not being at the covenantal heart of Judaism, nevertheless played some role in discussions of final judgment" 
"What is our hope and joy and crown of boasting before our Lord Jesus Christ at his royal appearing? Is it not you? For you are our glory and our joy.’ (1 Thess. 3.19f.; cp. Phil. 2.16f.) I suspect that if you or I were to say such a thing, we could expect a swift rebuke of ‘nothing in my hand I bring, simply to thy cross I cling’. The fact that Paul does not feel obliged at every point to say this shows, I think, that he is not as concerned as we are about the danger of speaking of the things he himself has done – though sometimes, to be sure, he adds a rider, which proves my point, that it is not his own energy but that which God gives and inspires within him (1 Cor. 15.10;
1.29). But he is still clear that the things he does in the present, by moral and physical effort, will count to his credit on the last day, precisely because they are the effective signs that the Spirit of the living Christ has been at work in him."  Col.
Add to this, Wright's open denial of Christ crediting his perfect obedience to Christians (imputation) and it seems we got a problem. Wright states:
What I do object to is calling this truth by a name which, within the world of thought where it is common coin, is bound to be heard to say that Jesus has himself earned something called ‘righteousness’, and that he then reckons this to be true of his people (as in the phrase ‘the merits of Christ’ 
In Piper's theological system, he also believes in the necessity of Christian good works at the final judgment, but he can describe these good works as "evidences" or "fruits" of truth faith and not something that "earns" justification because in his system, Christ's has already perfectly obeyed the law and thus "earned" and is the "basis" of the Christians' justification. Piper commenting on Romans 2:6-10:
(I)n general, there are two possible answers to this question. One says that eternal life would be based on perfect obedience if anybody had it. But nobody does, and so the only way to eternal life is by faith in Christ. The other way says that God never promised eternal life on the basis of good deeds, but always makes good deeds the evidence of faith that unites us to God in Christ, who is the basis of eternal life.
The other answer would say, it means that God does indeed give eternal life to those who persevere in obedience not because this obedience is perfect or because it is the basis or the merit of eternal life, but because saving faith always changes our lives in the power of the Holy Spirit so that true believers persevere in doing good. In other words, a changed life of obedience to God's truth (verse 8) is not the basis of eternal life, but the evidence of authentic faith which unites us to Christ who is the basis of eternal life. 
So within Piper's theological system, Wright will always be a "semi-Pelagian", no matter how many times he states that Christians are saved by grace. But it's important to note that in order for Piper to make Wright fit into his theological system, he needs to first prove that God requires perfect obedience from all men for salvation, which I believe is a very hard thing to do .
 Wright's Romans commentary pg 440
 Wright's essay “New Perspective on Paul"
 Wright's essay "Paul in Different Perspective"
 John Piper's sermon, "The Final Divide: Eternal Life or Eternal Wrath, Part 2"