Alexis,
IMO, leaving a negative rating for a customer is absolutely unprofessional. I would never, EVER do it and I question anyone who has said that it's crucial for the success of their shop.
I'm a HUGE amazon shopper, and generally people like to review things that are either amazing or awful. But, comparing etsy to those sites is like comparing apples to oranges. There's a whole different mentality here. Customers appreciate more and sellers work harder to impress. I hardly think that all of a sudden this new system will force customers to be more critical. I'm not even entirely sure that they'll notice it.
When i'm shopping or browsing, i'm constantly making snap decisions. These decisions are from things like pictures, wording, price, descriptions, star ratings, etc. Seeing a high star rating immediately makes me feel more comfortable. I think the stars will persuade customers to buy...