On one hand I understand why DN might not want to implement a downvote, on the other hand I'm frustrated at the low quality articles that make it here.
Instead, I'm proposing to have a MOTD (Message of the Day) style voting for each story, namely that both upvotes and downvotes are shown. This will allow dislike of a story become more vocal while presumably avoiding downvoting abuse that DN seems to be worried about. Given the lack of people here that actually vote, the stories would still be ranked based on the upvotes rather than downvotes.
EDIT: This would also make it clear when a person is using a voting ring on a story.