The thing with Piper is that he has written and preached some of the best material I have listened to or read. He can inspire someone to go deeper with God, but he can also step over a line and go where Scripture doesn't go.

Anyone who has followed John Piper over the years knows that he can and will rebuke people (as he did with Rob Bell). But he can also be undiscerning when it comes to people like Rick Warren, who likes to talk through both sides of his mouth; who can feel at home speaking to Calvinists and Arminians.
John Piper's only real complaint about Rick Warren is that he believes Warren is a little too pragmatic for his own good. The rest he believes his critiques are wrong about Warren.

Unfortunately Rick Warren is not the only person whom Piper has backed despite warnings by other Reformed leaning pastors and theologians.
Potty mouth Mark Driscoll comes to mind here.

I think that regardless of whether or not Piper is well intentioned by coming along side of people like Driscoll and Warren; I believe it is affecting Piper in a negative way.
Not too long ago John Piper made a plea to Calvinists. He wanted a “kinder more loving Calvinism”.
I wonder if his actions with people like Rick Warren are what he meant by his plea?
I am all for checking our pride at the door when discussing spiritual matters; but it is not kind or loving to let wolves in the building.

Tom