Of course, the obvious answer is "They won't pay"; so why not ask for money from the consumers of the reviews? OK, it's still pretty idealistic, and people will still go for free, but imagine a whole gamified review platform: to read reviews, you need to spend credits. To earn credits, you can either spend a small amount of money, or leave reviews yourself. This offers an incentive to leave positive reviews—it gives you access to more of others' reviews. Of course, the big problem with positive reviews is that people usually can't even think of their positive experiences; so prompt for a small amount of personal information: "Where do you live? … OK, people in Chicago are looking for reviews of Thai restaurants. Have you been to a Thai restaurant lately?", &c.
Your review will be decorated with an Amazon-style "This review was helpful" thumbs up / thumbs down button; and, the more helpful reviews you leave, the more weight your later reviews will be given. (The main problem that I have with review sites is companies posting spurious reviews of their own products.) Have comments be the last word, or allow threaded discussions (without moderation, or with moderation only for content violations); I'm perfectly happy with allowing businesses to respond—there are definitely unreasonable companies out there—as long as the whole exchange is out in the open.
Sure, the revenue stream from people willing to pay to read reviews is fairly small, but, if you build up a trustworthy review site, then surely people will come to it in preference to the current crop of useless ones out there, and the ad impressions will start to mean something.