I think the only "risk" of moving away from upvote/downvote (or at least publishing some type of suggestions for how they ought to be used) is that you'll start seeing more diverse opinions rise to the top of threads; and folks with majority opinions will have to engage instead of the drive-by-downvote. The positive feedback cycle of compounding diversity should be easy to imagine; and the chilling effect of downvoting as it exists today is a well documented bug in our reality.
As for the costs, even if all you could bring yourself to do is to separate downvote into "disagree" and "this is low-quality content", but scored them both just as you score downvotes today, you'd be providing better feedback to commenters as well as raising the quality of the discussion. Today the downvote (here and in so many other similar communities) is a huge contributor to the groupthink driven division-without-discussion that's poisoning our society.
To lower the cost to essentially zero, just turn off downvoting for a while and see what changes. If you're willing to experiment with cutting off entire topics, why not experiment with the structure of discussion?
I think there is even a strong case that HN's current policies are inconsistent. From the HN guidelines:
> Please don't post shallow dismissals, especially of other people's work. A good critical comment teaches us something.
But isn't that the essential function of the downvote? I'd much rather have someone say, "This isn't interesting to me" than just get the downvotes.
> Please don't comment about the voting on comments. It never does any good, and it makes boring reading.
OK ... but why is there nothing saying "consider a thoughtful, constructive critique instead of a downvote; use downvotes only for _________". What actually is the purpose of the downvote? Why is it a feature of this discussion tool? Was it included thoughtfully, or just because HN is a Reddit clone? Ironic that PG launched HN to be a "better" Reddit, but then you have in the guidelines:
> Please don't post comments saying that HN is turning into Reddit. It's a semi-noob illusion, as old as the hills.
HN suffers from one critical weakness that Reddit pioneered - the downvote. I'll end the comparison there; HN isn't turning into Reddit; but it has an opportunity to pioneer this type of threaded discussion and really differentiate itself from Reddit; if you're sensitive to the comparison, eliminating or improving upon voting is how you'll free yourself from that complaint.