1-star reviews are a treasure trove of customer pain points. When a pattern exist, these reviews highlight potential business opportunities. It'd be interesting if there were a website that collated 1-star reviews for common business / product categories from sites like Yelp and Amazon. Looking at these reviews could both be entertaining and hopefully inspirational for entrepreneurial readers.
Reviews written by people who are actively dissatisfied are interesting because they represent the perspectives of people looking for new solutions to their problems. This makes them ideal first customers, since they have a clear understanding of what they'd like a product or service to do for them and they also have not found one that does that. After all, if you're only mildly unhappy, writing a review is unlikely to be worth the effort unless you think you'll be getting a refund.