Hmmah, let's give this a serious shot.
First, we need a situation. Let's say, after accidentally wandering into OT, Thunderfall banged his head against the desk repeatedly, and knocked himself into a coma. In defiance of Modly conventions, the mods meet up and get on a plane, which unfortunately happened to be Oceanic Airlines Flight 815 out of Sydney. The good news is that Thunderfall set up a dead man's switch, and since he's in a coma, the switch hires someone to maintain and moderate the fora. The bad news is that he's a lazy SOB, and limits himself to making sure the forum's online and relatively bug free, and coming through every couple of days to clear up commercial spam.
Well, for maybe a business week, things would remain normal, with perhaps slightly more sniping than usual. When people finally figured out the situation, there would be a rash of "where have they gone!" threads, and allot of threads targeted at unpopular posters, getting more overt as it dawned on people that this was permanent. As this happened, allot of people would leave, either because they couldn't take the chaos, or because they're friends couldn't. Actual fans of the civ games would probably flee to Apolyton, although insular sub-communities, such as the NESers, would have a better attrition rate. Due to an evocative name that easily comes up in Google, newbies would probably continue to flow in looking for help though. Most of the remaining veterans however would probably be the ones with a presence in all of the forums, as opposed to just OT or Civ4's modding community.
Informal moderation by means of shouting would probably still occur, with the elite banding together to support eachother on these issues. Said elite would be a homogeneous group most likely, as black sheep would be shouted and bullied out. Allot of this "moderation" would be based on powerful member's personal pet peeves. So for example, the ban on discussing the BNP might well stay in place. There would be a heavy newbie attrition rate, as they overstep the informal boundaries of the community and got hounded out. All in all, even after the initial shocks, the forums would still gradually loose active members.
Allot the members of various forums who left right after the Event would be remembered fondly, and most veterans would fondly discuss how wonderful the good old days were. Of course, they do that already. Images with expletives would probably be favored as a way of getting around the auto-censors still in place, and important thing that need stickies (like directories for modding forums) would probably be formed either by taking over existing, inactive ones, or constant bumping.