I don't think 'like only' reduces the breadth of communication that much more than simple like/dislike does already. Especially if only the net difference is shown (1000 upvotes - 999 downvotes says a lot more than 1 vote.)

When I see articles upvoted on HN I'm never quite sure if the number of votes is telling me 'good article', 'raise your awareness of this issue' or 'good discussion about this in the comments'.