By general forums, do you mean forums that allow anonymous posting (versus requiring an account) or do you mean sites that have a broad topic (versus a narrow focus like HN)?
broad topic/community. Vandalized might have been too strong a word. Just mean that the broader the topic and community, the more likely it is to attract spam, ridiculous posts, etc. and for your case, ridiculous voting.
I wouldn't say that HN is necessarily all that narrow of a topic (it is somewhat open, even though it is primarily tech entrepreneurs), but the community is more narrow.
