Because of the power-law distribution, on sites with substantially lower traffic than Amazon, it's very possible that the helpfulness question won't be answered enough to be useful.
How do you incentivise someone to answer the "Is this helpful?" question?
How did Amazon manage it, even? The article talks about how subtly placed the question is. Yet I know I've answered it - to thank the author of the review, or to passive-aggressively express my displeasure at the wasted ten seconds I spent reading it. I also get annoyed if people downvote reviews I spent a lot of time on because they disagree with me.
Could you assume a review was helpful because you saw that a user had read it and then purchased the item? Maybe that would help populate the rankings, albeit crudely, for sites with lower traffic.