There are so many reasons, but you really need to do some research. Do a Google search on Rick Warren, or Purpose Driven, and see what the opposition has amassed in information.
Many of us have studied this for years. Here's a good start for you: http://www.newswithviews.com/PaulProctor/proctor85.htm
read any number of his columns.
Better yet, try wikipedia. there is a whole list of critiques at the bottom of the page on Rick Warren, to start you off.
I would also suggest researching Jim Wallis of http://www.sojo.net and see how similar his gospel sounds... and look at his political views.