They're sorted by a weird combo of recentness, upvotes, and lack of downvotes.

There's no way to tell if a 1-hour-old comment has more upvotes than a 30-second-old comment positioned above it on the page.

There's no way to tell if a single comment in a subthread has 0 upvotes or 100. This is the biggest problem, IMHO. Factual statements have no visible score for correctness.

There's no way to tell if a comment was upvoted for being interesting, or factually correct.

There's no way to tell if the person who upvoted your comment is the same person you replied to.