It is a broad term that ought to apply to any Christian who believes that the Bible is the inerrant word of God. I use it to describe myself, even though I am not a legalist or a KJV onlyist or a baptism by immersion onlyist.
When used in a media context it is often used as an attempt to smear Christianity in general with a vision of toothless, stupid, illiterate snake handlers and Holy Rollers. IMO Christians ought not to join in the denigration of Christianity by calling everyone who has a more conservative view of scriptures than themselves, fundamentalists. We are all (or at least we all should be) fundamentalists.
Truthfully, I know that Rick is far more conservative than he says publicly.
I suspect that Warren's problem is most likely tied to the fact that since he has become successful in his ministry he has surrounded himself with yes-men who nod at his every statement and encourage him to be non-confrontive. Being nice and being stupid are not signs of apostasy. They are perhaps signs of compromise. Who among us is not guilty of that?
Sometimes, I think they use fundamentalist to describe anyone who believes in the death and resurrection of Jesus as real things...
If I had to guess, I'd say he's exhausted all his own ideas, biblical knowledge and education. After becoming suddenly successful in his original area of expertise (if you can call it that), he has surrounded himself with advisors and sycophants who have groomed him for mass-consumption - telling him what press releases to make, what social projects to pursue, and what market segments to appeal to, to increase his influence and, market share.
Except among the fundamentalists, of course. Rick's made it plain he doesn't want our money.
"Fat, drunk, and stupid is no way to go through life, son."
One can not be a Biblical conservative and cavort with, and/or promote catholic mystics, new agers, contemplatives, false teachers (Cho, Schuller, Manning, Nouwen, Mclaren/emergent church leaders, and etc., etc. etc.), jewish synagouges and so on.