This is a great idea. The one potential UI issue to be aware of
(which I encountered in the old ACES interface for bboard alerts)
is how to handle nested relationships. For example, a user can
set email alert preferences on a thread, one bboard,or all
bboards. Do you allow a user to, say, set immediate emails on
one thread but also get a digest for the whole bboard on a
weekly basis? If so, how do you represent those choices to the
user? And how do you handle cascades when a user changes
preferences at a higher level?