It's no secret that required registration lowers conversion rates for ecommerce checkout. According to Forrester Research, one in four customers will leave a site if asked to register before purchasing.
Why would it be any different when it comes to product reviews? Most retail sites require registration before submitting a review. How many words are being left on the table?
Altrec is one site that lets you quickly and easily, all you need to enter is an email address (similar to leaving a blog comment):
The only downside to this is if you have a lot of reviews to submit, you need to enter your name and email address each time.
Though it's tough to attract customer reviews, shoppers trust the opinion of "people like them" far more than your slickest sales copy. Customers are willing to enhance your product pages out of the goodness of their hearts. Make it easy for them.
If your reviews vendor does not allow you to offer this to customers, suggest that they make this a high priority in their next product release. Their business model is enabling customer feedback -- so go ahead and tell them what you think!