Including a List-Unsubscribe header in your emails will reduce complaints, improve deliverability and improve the experience for your subscribers. It's easy to do and doesn't cost anything.
It will reduce complaints because your recipients will be able to easily and reliably unsubscribe if they want to. Frustrated users are likely to hit the "Report Spam" button or complain in some other way about the email that they requested, hurting your sender reputation. In Windows Live Beta, when you enable the Unsubscribe link the Report Spam link is not an option.
Including a List-Unsubscribe header is viewed positively by most ISPs and spam filters, and allows unsubscribe reputation services such as Lashback to more reliably monitor your unsubscribe compliance. User feedeback (the "Report Spam" button) is one of the most significant factors is deliverability at the major ISPs such as AOL, Hotmail, and Yahoo.
There has been a lot of "urban legend" around whether or not you should click on unsubscribe links in email messages you receive. Many ISPs warn against using any unsubscribe messages in the onilne help for their users. The words "click here to unsubscribe" can still set off some spam filters! Using a List-Unsubscribe header provides a standard way for users and software to know how to unsubscribe. Having an Unsubscribe button show up instead of a Report Spam button will make a big difference in how your recipients perceive the messages you send and your organization.
Most importantly, this is drop dead simple to do. Most software packages support it as an option if its not there by default. Most others will allow you to add "custom headers" pretty easily. To start, a generic unsubscribe address such as "email@example.com" is better than nothing - but eventually you should try to provide dynamic email addresses that are more dummy proof such as "unsub-listname-12352B@domain.com" (read more about VERP)