i think it's good solution (in lots other shops i see that) is to give possibility via CLients/guests to write reviews/opinions (sometimes people know product / or want to do that) like on guest.
Nopcommerce support this. Great, but...
Feature about i think is add information or "Review wrote by Client who bought product" or smth. Like that. It's small change but can boost quantity of reviews and they quality. Of course it's good to give possibility sort only "who bought" and "people without bought".
What you think?:)