You wouldn't have to apply the sentiment factor for all sites. You could have a rule that would only lower rankings for sites which have an overwhelming majority of negative links (say >90%). I don't think there would be many cases where a spammer could achieve that level of impact and those few cases could be dealt with through manual intervention.

As for determining sentiment, it's not something I've ever tried to do, but is it really that hard ? Intuitively I would think positive and negative articles would have significantly different distributions of certain words.