I'm late to the party, but happy this is finally getting implemented after almost 10 years! The date of the first request for this functionality was in 2004 I think!
Instead of elaborating features at the configuration level (Moodle is so complex, it's hard to know what it means anymore), I'd suggest focusing on the use cases that should be supported. You already gave a couple in your email (Jeff's use cases). I'll throw out two more:
- Student in a class with 2000+ students asks a question in a general forum with high traffic, and only wants to receive emails regarding her question (thread-subscription).
- Student lurks in a forum about Homework #1 because he's having trouble with it, and decides it's a good thing to subscribe the whole thing.
Cheers,
C. Fuhrman