An attacker creates a large number of nodes and overwhelms any signal in the initial system.
For the specific example of a reddit-based forum, it's trivial for an attacker to make up a sizable proportion of assigned reputation points through the use of sockpuppets. It is only moderately difficult for an attacker to automate the time-consuming portions of this process.
Limit the ability of low karma users to upvote.
This thread is for asking any questions that might seem obvious, tangential, silly or what-have-you. Don't be shy, everyone has holes in their knowledge, though the fewer and the smaller we can make them, the better.
Please be respectful of other people's admitting ignorance and don't mock them for it, as they're doing a noble thing.
To any future monthly posters of SQ threads, please remember to add the "stupid_questions" tag.