This addon provides very fine grained control over which site may or may not interact with another one. It can serve a handful of purposes, including the prevention of tracking.
The downside is that you have to build your own whitelist, and it requires time, and also a reasonable amount of intuition/knowledge to tell apart legitimate 3rd party domains (CDNs, APIs, etc.) from others. Just like NoScript I would say.
Fortunately this addon is most welcome because it fills a gap NoScript and adblockers failed to fill. Let me give an example: I sometimes want to permanently disable my adblocker for a given site; doing so allows legitimate ads but also tracking snippets from social networks such as Facebook Like buttons, which Fanboy's List would block otherwise. With RequestPolicy I'm now able to allow ads on that site while still blocking Facebook buttons. NoScript doesn't have this 3rd party aspect.
Like said in the description, RequestPolicy and NoScript complement each other and allow for a very good privacy and security on the Web.
Definitely worth 5 stars, even despite some minor flaws. I'm sure they'll get fixed soon :)