XenReviews is an add on for XenForo that adds a new section to your site. The administrators can pick who's allowed to create items and who's allowed to review them. Items can be anything products, stores, locations, people, anything you want.

You can have an unlimited amount of categories and sub categories. Each category will have custom fields that can be sortable (and possibly filterable).

There will be plenty of ways to make money with this add on. If applicable you can have an affiliate link for a "buy now" button. Users will also be able to pay to have an item listed as featured or paid (this will have the ability to be disabled).

There will be plenty more, all features will be listed on the demo/development site once it gets set up. Until then the plan can be seen here: http://xenforo.com/community/threads/possible-review-add-on.61174/

With a specific outline there won't be any risks or challenges. The most important thing is everyone giving feedback and specifying what exactly they want.