1. Let anyone comment on other people's comments - like HN or Reddit, with similar reputation (points) and up/downvotes.
Right now, unless an owner monitors and responds to every negative comment, there's no recourse for being unreasonable or flat-out inaccurate. Even when an owner does so, the recourse is minimal. Let any other Yelp user reply, turning each comment into a thread.
2. When someone posts a 1- or 2-star review, show a second, required comment field for "What happened when you informed the retailer?"
If someone is served a meal they don't like and says nothing at the time, they skipped a - the - critical step. While there are cases where a low review could be justified without ever giving the retailer a chance to address the perceived problem (like if someone showed up twice during posted business hours and the retailer was closed), they cause fewer than 10% of 1- and 2-star ratings and they're easy to explain. Otherwise, the minimum for a negative review to be constructive is having informed the retailer (and let them try to address it).