To me, one of the unique things about LAS is getting both positive and negative feedback. I don't know that I'd participate if it worked more like other challenges with a poll, or a basic ranking system, etc. To me, LAS represents a rare opportunity to to hear about what people don't like about what you're doing, honestly and anonymously, and this is one of the draws of participating, for me. (Even if it might not be the most pleasant of experiences, having to read negative comments. ^_~)

So yes, I do think negative feedback is useful, as long as the voter provides a specific reason for the vote (other than something like "everything was bad, but this was the worst").

If you had to give feedback on all the stories in a round, say 12 stories, would this put you off voting? - Yes, it would. I think this would be asking too much of most voters, and could possibly reduce the quality of the feedback all around.

So yeah, for me, I think the current system works.