A couple of ways. For one, you were able to set up filters to filter out anyone below a certain amount of feedbacks or anyone with X number of negative feedbacks from bidding on your item. Obviously by eliminating the ability to leave negative feedback to buyers eliminates this.
You could also cancel the bids of buyers and offer the item to a different bidder if they violated the terms of the sale (or didn't pay). As absurd as it sounds, someone can bid on your item, not pay, and still leave you a negative feedback. For example, they place the "winning bid". After the auction ends (and before they pay) they contact you and tell you they want the item shipped to Nigeria. Even if Nigeria isn't listed as a place you sell too, and you explicitly point out in the item description that you don't ship out of country, they can still leave you a negative feedback - all without paying! In the past buyers were hesitant to due that, because they'd certainly receive a negative feedback in return. Now they can be as scummy as they want without consequence.