Eh, you could sort them higher if they have a ton of discussion under them. I think that could work pretty well. Maybe prioritize number of direct child comments over number of descendant comments (i.e. one long comment chain between two people shouldn't push it to the top).

If you think a topic is interesting, you'd comment under it. That says nothing about whether you like or dislike the comment, it just means that comment provoked some kind of response. That's also not perfect because there are plenty of times where I don't think I have anything valuable to add, but maybe it's an interesting metric to try.