In my opinion certain items can require a lengthy review, such as technology or items with lots of features. But some items like screen protectors and cases don't. 250 words for A+ seems high for me and I've had sellers requesting a minimum character limit. As a regular amazon purchaser and reviewer (not from amz), I find that if a review is forcibly long and paid for I'm less likely to believe it. I'll look to the non-paid reviews too. A phrase I have grown up with springs to mind "Quality Over Quantity".
So what do actual sellers think of requesting minimum word counts and basing the rating system on the length of a review?