John Piper signed the Statement of Concern about the TNIV Bible. In a recent interview with World magazine, he stated many of his reasons:
In a nutshell, my view of the English language Bible translation enterprise is that the goal should be to preserve the ambiguities in good English that are present in the original language. Paraphrasing these ambiguities in English to "clarify" them is the role of the preacher and commentator, not the role of the translator. So "Jews" should not be paraphrased "Jewish leaders." The "obedience of faith" should not be paraphrased "faith and obedience." "The freedom of the glory of the children of God" should not be paraphrased "the freedom and glory of the children of God." "Bears all things" should not be paraphrased "always protects." "Who hopes for what he sees?" should not be paraphrased "Who hopes for what they already have?" "Saints" or "holy ones" should not be paraphrased "God's people." "We are being killed" should not be paraphrased "we face death." "Choosing to show his wrath" should not be paraphrased "although choosing to show his wrath." And the word "because" or "for" should not be omitted 30 times in the book of Romans.
Many of the changes in gender language are also problematic. As stated in the Statement of Concern, "The TNIV makes significant changes in the gender language that is in the NIV. The TNIV raises more concern in this regard than previous Bible versions because, riding on the reputation of the NIV, the TNIV may vie for a place as the church's commonly accepted Bible. We believe that any commonly accepted Bible of the church should be more faithful to the language of the original."
Vern Poythress and Wayne Grudem, The Gender-Neutral Bible Controversy: Muting the Masculinity of God's Words
Vern Poythress, "Political Correctness and Bible Translation"