Personal insults can be quite difficult to describe, but are easily understood, and, when you are on the receiving end of one, (especially if it is laced with contempt), you are in no doubt whatsoever that you have been insulted, sometimes profoundly.
In any case, I'd see it somewhat in terms of distinguishing between the person and the argument. While arguments, or differences between points of view expressed robustly can occur anywhere on these fora, (the iPhone forum, which I have never visited, appears to give rise to strong emotions, feelings and expressions of differences of opinions), in PRSI, the hallowed halls of which I do frequent, arguments can also become quite heated, as people tend to hold opinions/views quite strongly.
Here, the key distinction seems to me to be to differentiate between the person, and the argument. It is acceptable to suggest that the argument (of someone with whom you disagree, sometimes strongly) is flawed, and may even be uninformed, profoundly mistaken, perhaps even bordering on a definition of daft, or breathtakingly dreadful. However, it seems to me that is not acceptable to call your opponent a cretin, or moron, or imbecile, or insane, or someone almost illegally stupid, much though you would love to, merely because you disagree with what you think are (sometimes with justification) their deeply disgusting, repellant and utterly unpleasant viewpoints.