Organization and availability would be big ones.
Per se an ISP wanted the list, they should be able to download it and install it in their domain blocking section of their email server.
Maybe an automated update like Norton? OoooOOOoo....that sounds nice!
And of course anybody that enters a domain in for blocking must make a case prior to it being added. This is where the tricky part is. Because the people behind the spamming domains will be forever trying to get themselves off the list. And if the site grew very large then it may also head down the legal trail as well.
So the domain listed by a person to be blocked should have to prove the case and the proving should be associated with the blocked domain and the domian liter as well as the domain that is blocked itself. (column and table thoughts..hehehe).