i have left negative feedback for a seller once and felt i was justified, so they turned around and left negative feedback for me in retalliation, ebay may want to stop some of that
After Brian's question last night I went to find that junk email from eBay that had mentioned their upcoming changes. This was the main part of what they were doing. If a seller can't leave a negative for a buyer it eliminates the retaliatory negatives that sellers leave if they screw up.
In reality it's a way to hopefully keep new buyers on board though. Their numbers seem to have peaked. And many who get a negative (especially early on) just never come back. So they're trying to make it less threatening/more friendly to buyers. Especially new buyers.