Bring back firstname.lastname@example.org. Non-Yahoo users (like myself) who want to help you shut down accounts used for spam shouldn't have to jump through the hoops of reporting through a form on your site. I used to be able to forward the original message + message headers to email@example.com - something I can still do with almost every other major email provider out there, and something that I'm sure they're grateful for.
You've just made it so that I won't report, and you won't know to investigate and shut down abusive accounts, making Yahoo less relevant in the long run.
Don Dinnerville commented
PS - requiring someone to sign in to provide feedback also sucks.