Downvoting things that you don't like. Around 15 years ago, when Reddit was very very young, downvotes were almost never used, except to weed out bad advice, ignorant replies, abuse, etc. As more people got in, the downvote button became the dislike button; with people even arguing that that was the original purpose of the downvote button. Replying with a link to the reddiquette got you downvoted even more lol.

Upvoting useless rubbish comments to the top.