The ability to mass PM would be great. I love the idea and am surprised it's not already a feature.
Email can get buried in a person's inbox or marked as spam, as stated.
As obvious as it is to some, a global announcement post on the open board can easily be missed or simply ignored.
The individualized nature of a PM makes a person take notice and read your communication.
As an aside, I've previously suggested the ability to create groups in the PM buddy list, which ties into this. (Would be great for contacting mods, certain groups of users, etc.)
