Online comments don't fall neatly into "positive" and "negative" buckets. There's a range of consumer sentiment that challenges even the most sophisticated natural language processing technologies. Catherine van Zuylen, VP of products at sentiment-analysis vendor Attensity, details at least seven shades of sentiment that defy easy categorization, from false negatives like "crying with joy" to compound sentiment like "I loved the trailer but hated the movie."
Sophisticated sentiment-analysis systems can be optimized to handle some subtlety, but no amount of tuning will lead to perfection. It's best to focus extra effort on acting on the majority of clear-cut sentiments.