Reddit has a horribly broken spam filtering system for comments and links, which frequently catches legitimate posts, and then people have to ask subreddit moderators to mark them as "not spam".
Why? Well, moderator abuse. Moderators can mark stories and comments as spam to get rid of them. But there is always moderator abuse. So the spam filter has got trained on tons of comments and links that aren't spam.
How would they fix it? Give moderators a means to get rid of bad posts other than marking them as spam.
this has been implemented (fairly recently). there are now both "remove spam" and "remove ham" options, the latter removing a post without using it as spamfilter input.
Why? Well, moderator abuse. Moderators can mark stories and comments as spam to get rid of them. But there is always moderator abuse. So the spam filter has got trained on tons of comments and links that aren't spam.
How would they fix it? Give moderators a means to get rid of bad posts other than marking them as spam.