Mark Brownlow talks about Yahoo's take on subscriber engagement here. After you read that, check out Mark's more generalized theory on what ISPs consider when determining whether or not email is unwanted. Comments from Yahoo's Mark Risher confirm what many of us already knew -- subscriber engagement matters.
Anti-spammers and deliverability people both get hung up on opt-in (alone) sometimes. A sender will say, this mail is opt-in, how dare an ISP choose not to deliver it. Various blacklists will harp on confirmed opt-in (alone) as the sole arbiter of whether or not mail should be delivered.
Truth is, they're both wrong.
Sure, opt-in matters. Your mail has to be opt-in, and confirmed opt-in is the best way to do it. If your mail isn't opt-in, all bets are off.
But, ISPs care about *more* than just that. They're figuring out whether or not recipients care about mail from any given sender. If the people on your list don't care about your mail, the ISP doesn't care about your mail, and that doesn't bode well for your ability to deliver that mail.