If your unsubscribe link asks me to type my email address before it unsubscribes me, I'm marking your email as spam.

One reason to do that: you forward a message to me, I click unsubscribe and unsub you.

I guess that's fair. Then it should pre-fill my email address. Otherwise I may not even know which of my addresses is subscribed!

A hairy problem. One solution: one-click unsub, with a huge “user@example.com has been unsubscribed. Didn’t mean to? Subscribe”