>>10 I agree with John McCarthy's saying "Don't defend your views; test them." Ideally, either I or my opponent(s) will be convinced, but even if that doesn't happen we will both likely be more enlightened as a result.

In my experience most internet debates seem to fail for a few reasons:

1. Fallacies are used and go unchallenged.
2. Facts are given without source, or assumptions are made with personal experience as their only justification. These are then argued about fruitlessly.
3. The participants tire of it.

With fallacies, the one I see most often by far is ad hominem. An anonymous discussion is definitely better in this regard, but is not immune to it. The bandwagon fallacy and argument from authority also often seem to me to be used.

When the participants get tired of arguing, often it's because they're writing massive back-and-forth letters (e.g. Person A makes 5 points, Person B refutes 3 of those points and adds a point of his own, Person A refutes all the refutations at once...), or there's name-calling and other nonsense going on (see fallacies). Often, it seems to me that people really are not ready to change their opinions, and would rather dismiss the other person's argument than seriously consider it.

I'm obviously guilty of #2 right now, but these are just my thoughts. Anyone else have any to share?