The fundamental issue is that the entire focus is how other Christians are false teachers and unsaved, while somehow he has the truth. Now obviously we all believe our own views are correct, but we need the humility to recognize that we may be wrong. Yes, we are to expose false teachers. But that doesn't mean that everyone who disagrees with us is a false teacher.
Luke 9:41-50 is a classic example.
Also bear in mind Romans 14. There are many things that we may differ on while still being brethren. Certainly for each of these issues there is only one true answer, and we should strive to understand it better, but it isn't necessarily a reason to reject someone who comes to different conclusions.
Remember also that we are not God and have imperfect knowledge (1 Cor 13:12).
Basically, there are some issues that are essential, and some that are not. Darwin Fish rejects this view (directly stating this), instead holding that every part of the Bible must be understood correctly, ie in the way he understands it, or you are going to hell.
In his search to understand the Bible correctly he has done some very interesting study, some of which may be valuable reading. But the basic assumption behind the site, that only following his narrow interpretation of every last detail of Scripture will bring salvation, is seriously false.