IIRC the individual posts are sorted by votes so upvoting should be sufficient
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.
For example: I use yelp to see when stores are open/closed and if a coffee shop has wifi. I could care less about the reviews, unless one of them happens to position a specific deal I'd be interested in.