Root meaning of the word "fundamentalism" appears to be just biblical literalist. Jehovah's Witnesses' fit the bill, even if there are other biblical literalists who don't even count them christians. By extension one could say that anyone with strong belief in the truth of specified propositions is a fundamentalist about them. But clearly there's nothing bad in strong belief. There's nothing intrinsically wrong in acting on a strong belief either. So why do people use the f-word pejoratively? Who are the bad fundamentalists?
-Traditional or conservative folks are not necessarily fundamentalists.They might even be private ironists. Sometimes it's good to challange them exactly by reminding them of principles and ideals.
-Activists who act on strong moral beliefs need not follow any rigid Party line. Sometimes party line can check counterproductive behavior.
-Intolerance and harassment may flourish without any kind of doctrine. They are often caused by mere selfishnes and opportunism. Public moral doctrine could be helpfull in our own over-permissive society.
-Literalists about some faith, doctrine or ideology need not be aggressive at all. It depends on the doctrine in question.
So what do people mean, when they use the f-word pejoratively? Usually the idea is that fundamentalism equals authoritarian, intolerant and aggressive behavior. We are supposed to think about animal rights activists or islamists here, people who challange the mainstream western society.