Right now everybody registered can rate the plugin how they want without giving explanation. Maybe there are something wrong with it or person just had problem installing it. We would never know.
This makes huge difference since community is huge, some users have development background can objectively tell when plugin or theme are of good quality, and there are some that might vote when something minor or specific to their setup is not to the liking.
When reading reviews on sites like Amazon, NewEgg and App Store it deeply sadden me that some users confuse the purpose of the reviews. The issues with shipping, installation, and understanding of description are often confused with actual product review, its functionality, quality and other attributes of it. These websites do a good job by requiring users to write their review when giving rating, that often helps for other users to distinguish when it actually item review or not. Many even have reviews of reviews, that is asking was it helpful or not.
WordPress made to be so great because of all people contributing, building plugins and designing themes. Everybody can come up with idea and realize it with wordpress that what makes it better than other blog and cms platforms. Because of this things it grew so big with more and more new users, we should help them to find useful plugins fast and efficiently without making them to go through all of them and trying to figure out why is this one is better than other. Or why does this one has 3 stars and 2 million downloads and another have 10k downloads and 5 stars.
I purpose to make it mandatory write review in order to vote on plugins and themes.