The only US english equivalent to "gormless" (no 'e', sorry) that i know is the phase "the lights are on but nobody's home". "Vacant" is used similarly, at least where I grew up. It's not particularly impolite, certainly less so than calling someone stupid .(posted 8331 days ago)I've no idea where it comes from. The only other "gorm" I know is the gaelic for "blue", which can't have much to do with it. We have quite a collection of slang dictionaries at home, so I'll see if I can dig something up, but don't hold your breath.
As for the forum, I think moderation helps, but an active slapping down of juvenile behaviour by the regulars has always been a major component of the better internet groups that I've been a part of. There doesn't seem to have been too much of that here, pace steve.c, so perhaps the forum just benefits from obscurity.
It would be interesting to know how many people lurk but don't contribute. The web logs for my images have downloads from people whose domains I don't recognise, but not in any great number.