That is also a good idea because people are motivated more to act when something goes wrong; and a close second is when they are excited to share a good discovery. The DefaultReviews [1] gist may offer some guidance:

"...rating by default is the equivalent of saying "everything is okay". That rating is made without you having to lift a finger. You only have to lift a finger to change it to "awesome" or to "sucks" or to add a elaboration for others to read."

Also, the final section of DefaultReviews [1] may also help solve your inevitable problem with faked reviews. Pull requests welcome.

[1] https://gist.github.com/iL3D/59df64947d42828d848ebfc1651a312...

Great! I have done some user research for a similar problem, and what users look for in reviews is mainly a list of potential problems, so that they can decide if they can deal with them or if they are dealbreakers.