Fundamentalism, whether Protestant or Orthodox, has essentially the same attributes, and manifests itself in essentially the same ways. So, when I speak in this post of fundamentalism I am speaking of both the Protestant and the Orthodox versions of this problem. Orthodox fundamentalism and Protestant fundamentalism are, really, nothing more than two sides of the same coin. Thankfully, though, the cure to fundamentalism (of whatever group) is the same no matter what group you are in.
I don't claim to have the exact definition of fundamentalism, but if I had to describe it, I would say that wherever fundamentalism is, Christian virtues such as humility, soberness, gentleness, patience, non-judgmentalism, love, and so forth are, to a greater or lesser degree, absent or distorted. In other words, the more fundamentalist one is, the less these virtues are able to spring from the soul of a person and manifest themselves in outward actions. Fundamentalism, then, is what happens when someone made in the image of God becomes spiritually ill in a rigid and self-righteous way.
The main root of fundamentalism is the mindset: "We're right, you're not." This mindset is the exact opposite of the patristic view of humility, which teaches us to consider everyone as good as--or better than--we are. As Christians, we are taught to second-guess our opinions, and to not rely on our own judgment as to what is correct and incorrect. Not before we have cleansed the nous, anyway. If someone does correct another, it should be because they have been purified, and because they have the authority/role for doing so (e.g., a Priest correct a Parishioner, or a Father correcting a Son).
But the root of fundamentalism makes people think that anyone and everyone has the right to correct and contradict others, based simply on the premise that they have correct information and that others are wrong. The degree to which someone has been healed of their spiritual sickness is irrelevant. All that matters is that fundamentalists are correct, and have information that they think others should have (whether the people want the informration, or really need it, is irrelevant).
Fundamentalism has a certain narrowness of vision, an inability to see beyond the limited scope that has been chosen by it's adherents. This lack of perspective is what the Scripture speaks of when it warns us to be sober, for fundamentalism is really intellectual laziness, and the letting down of the guard of one's heart. People are sometimes so busy attacking others that they do not realize that they have let the gates to their own heart open. Fundamentalism is letting oneself off with the easiest (and many times self-congratulatory) answers.
Often a fundamentalist seems very knowledgeable about what they speak: exactly because they speak in such a narrow way and have studied in such a narrow way. By not having to worry about love, gentleness, etc., and by having spent so much time debating a small selection of topics, they have been able to spend all of their time sharpening their apologetical arguments, and fashioning like some word-smith the most persuasive rhetoric.
Thus, fundamentalism can many times sound very logical and correct. The fundamentalist proof texts in such a way that what he is saying seems like the only logical end to the situation or doctrine being talked about. Of course, the fundamentalist (many times unwittingly) rarely gives all the data necessary to come to the proper conclusion: but one (especially if they are inexperienced or uneducated on the matter) can get so caught up in the rhetoric that they totally miss the fact that much information and perspective is lacking. So when the fundamentalist gives proof texts from what he claims to be infallible sources, this can sometimes be very persuasive.
The fundamentalist, in fact if not in words, claims to be an infallible interpreter. Of course, what the fundamentalist says is something like "It is obvious to everyone," or "The truth is obvious," or "Anyone can see what is right here, let those who have eyes see". They claim to be infallible, but hiding under a cover of humility, say that they are really not doing anything other than recognizing what should be obvious to anyone and everyone. This is the way all fundamentalists works, whether Protestant, Orthodox, or of whatever other group. And such people really cannot comprehend that you do not see the "obvious" or "common senese" or "plain" conclusion or fact that they see.
All of this is very attractive to people who are given to extremes, and especially people who wish to see the world in terms of black vs. white, with as little grey in between as possible. Young people, or those new to a group, are the most susceptible, because they lack both the experience and the learning to spot the errors in the fundamentalist mentality. Normally it is both the young and those who are new who have the most mis-placed zeal, and are most able to go astray because of a lack of patience.
Fundamentalism supports the notion that people can know it all and get it all right, with very little effort. Fundamentalism is a prideful pat on the back that says: you really are as smart and right as you think you are, and all those other people who disagree really are wrong and have gone astray. But, where Christian virtue would have us be kind and gentle with those who have gone astray, fundamentalism does the opposite: making people angry and bitter towards those they deem to be wrong. This animosity is sometimes hidden, of course, but it is there nonetheless and will manifest itself eventually.
(An aside--Am I the pot calling the kettle black here? I guess it is. I apologize for this post being hypocritical. It's just something I had to get off my chest. This was, above all, introspective.)
The cure to fundamentalism is reading the Scripture, the Fathers, reading and imitating the Lives of the Saints, going to the services of the Church, participating in the sacraments, loving one another, being humble, doubting one's own opinions, being patient, giving alms, and being open to correction. Above all though, one must be freed from the narrow mindset of fundamentalism, because otherwise all the patristics and scripture readings and whatnot will get filtered through this mindset, and like a funnel will simply lead all the information back into supporting the small set of beliefs focused on by the fundamentalist group.