You may wish to consider whether individual objects notify all other objects, or a selected list of objects, or objects of a specific class.

Something to watch for in the "notify all" scenario is that the notification doesn't trigger another notification of the same type by the original object - that way leads to great activity with no forward motion. You may wish to tag notifications with a sender ID as a partial solution to that problem, although if another object responds to the notification by doing some work that notifies the original object (perhaps indirectly!), then you still have a problem.