I quite like discussion forums. I frequent a fair few of them, and I learn a lot from them. I also get accused of being a hostile person in them, from time to time. Although it is true that I can be very abrasive, I'd like to imagine that I am hostile only in retaliation.
The thing is, I believe that what many people consider to be "polite rules of discourse" provide an unfair advantage for people who get to claim a "common sense" opinion.
It's easier to pick up on the hostility of a sharp barbed comment than the one in a smarmy, disingenuous one, that only insults by implication, but it's still there.
A classic example would be "just asking questions". It's a thin line of course; you don't want to chastise someone for having and making a "dumb" question. Occasionally, though, you can see people trying to weave a web. Asking for a clarification, then acting dramatically surprised so as to in-turn portray it stupid and insufficient. Deflecting with excuses that don't stand up to scrutiny but add attrition to a read, that the asker and any reader could've answered for themselves with a second's introspection.
Basically, behavior that indicates, not that someone interested in understanding a situation, but that someone is trying to preach their opinion while disguising it as a learning process, taking advantage of a widespread predilection for politeness.