Community-building seems to be on a lot of people's minds these days. In the Internet commerce sphere, its importance was acknowledged from the very start. Here are some examples of engaged user communities, some focused on product reviews and some not.
Tirerack.com. They sell tires. And wheels. Not much else. Their customer feedback mechanism is much more detailed than just a star rating: after you buy tires, they allow you to rate wet traction, dry traction, snow and ice, etc, all separately. And with dozens of user ratings on every tire you might consider buying, they're the definitive source of real-world information.
Newegg.com, electronics retailer. Their users include real experts (that is, people just a little smarter than you) whose opinions are invaluable. Newegg's service is good enough to keep their customers satisfied with the retail part of the experience, so they don't have to be afraid when those customers return and comment on the products.
Songmeanings.net, song lyrics database. I've become quite fond of this site, but then, its competition scrapes the bottom of the barrel. The conversation here is mostly quite thoughtful. I've found surprises like the theory that Interpol's "PDA" is about date rape, and sincere but amusing nonsense like Moving Clocks Run Slow by We Were Promised Jetpacks being about Einsteinian special relativity.
Epicurious.com, recipe database. It's a Conde Nast site, so it contains all the recipes from Gourmet, Bon Appetit, and several others. The wonderful thing is that users who rate recipes are always careful to mention any changes they made, and how they might change it if they made it again. This way, you don't have to look at an unfamiliar technique or ingredient and wonder how it will work out - you can see how other people dealt with it. Other sites have made similar achievements in this space: CookingLight.com, whose database is also contained in MyRecipes.com.
DIY sites covering loudspeaker and amplifier building are great guides for their users, partly because people stick with hobbies for a long time. And of course Linux, wikipedia, etc. were built by users and have pretty much become institutions as a result.
Engagement is power, I think. The people putting their energy into these communities are amateurs - with very few exceptions, they don't make a living at whatever they're contributing to. But their collective efforts build something with a life of its own. For a retailer, that means return business. For a hobby, it means people being able to accomplish things they otherwise couldn't.