Comment voting. Unless I'm gravely mistaken, it's been going pretty smoothly so far. It doesn't do anything other than point out humorous or informative comments for now, but that's not what I'm getting at.
Downvoting on a comment is supposed to be reserved for things that are inappropriate or in bad taste, right? Hence the "Are you sure you want to mark this comment as inappropriate?" dialog box when you click down.

Not that it's been abused, but I think it's being misused.
For example, look at this thread.
Chances are if a comment has any votes, it also has downvotes. People are (from what I can see) marking comments they simply do not agree with as inappropriate.
If in the future comments are deleted because of their being inappropriate, many comments that are downvoted now will be discarded simply because someone didn't agree with the opinion set in it.

I think this is an issue that needs to be dealt with before it becomes a real problem.