Reddit has an option to sort on controversial on a group and also on comments. I suspect the algorithm is probably looking for things with a combination of upvotes and downvotes. Take a look at any reddit group or the comments and sort on controversial if you want to see what it looks like. If you defined controversial as any thing that has a 15% to 55% down vote ratio that would be fairly specific. Maybe over 60% downvote it may be off topic or spam? It would be an interesting experiment, for a few days, to change the default from "hot" to "controversial".