Same as Reddit. Accumulate enough points via posting shallow and uninteresting—yet popular—dialogue to earn down voting and flagging abilities, which can be used (via automation) to manipulate discussions and suppress viewpoints.

Slashdot's system was superior because mod points were finite and randomly dispensed. This entropy discouraged abuse by design—as opposed to making it a key feature of the site.

It's the Achilles' heel of Reddit and every site that attempts to emulate it.

Critically, Slashdot also had a meta-moderation system, where users were asked to judge moderation activity to confirm whether it was sensible, fair, and so on. I'd like to believe that system played a vital role in stopping abuse of the moderation system. It was way ahead of its time.

I've been advocating for a while now that HN could use meta-moderation at least on flagging activity, so it can stop giving flagging powers to users who are using it for reasons other than flagging rulebreaking.

Reddit awards one karma for a comment if it doesn't get downvoted. I noticed the other day that I got a pretty random and only tangentially relevant comment on a one month old post I made. I checked out the user, and they were only commenting on old posts to slowly accumulate karma. Only the poster will be notified about such a comment, and as long as it is made to be made of platitudes, most people will not bother downvoting.