I append some links.
On balance, I'd suggest subscription at the discussion level is a great enhancement we really need. How about voting for it? http://tracker.moodle.org/browse/MDL-1626
Some of the discussions and comments on subscription at the discussion level.
These range from politics/philosophy (you are selfish if you want to not follow everything) teacher management issues (How to deal with forums, you can choose to make some threads compulsory subscriptions . . . ), coding difficulties (to do with overheads), code that does work but has not been accepted into the core, huge numbers of votes for this issue . .
Subtle issues like: Do you have to GO there to a forum to subscribe or do you have a setting "Subscribe to NEW threads" etc. then selectively unsubscribe.
1. The Moodle feature tracker: http://tracker.moodle.org/browse/MDL-1626
2. Discussions: http://moodle.org/mod/forum/discuss.php?d=118619
3. The discussion about 'spam': http://moodle.org/mod/forum/discuss.php?d=9960
4. The heated discussion where posters accuse others of being selfish:
I quote: "It is my understanding that as a FREE COMMUNITY based project the idea of MOODLE is for evrybody involved to SHARE their ideas. It seems to me a function allowing for subscription to single threads would only be useful for the shamelessly selfish. That is, people who only want to see the repies to their OWN threads and those threads in which the INDIVIDUAL is personally interested. For those people a 'Please help me with my problem, and by the way, don't waste my time with other people's questions..' kind of facility would be perfect. But I think that for most of us interested in learning and sharing via the very commendable social constructionist approach and open source principle it isn't really relevant."
Probably the best concentration of the issues.
http://moodle.org/mod/forum/discuss.php?d=9959
5. Some comments on Forum Management:
http://moodle.org/mod/forum/discuss.php?d=10070
No doubt there is more in the forums, and other issues I've not considered here . . . I lived with a system for 8 years with subscriptions at the forum and thread level and it was cool. Moodle seems inflexible in comparison. I 'monitor' 20 courses (suitable for digests), I am active in 6 courses (need post by post for some, by the thread for others) and dabble in 20 more . . . the system wide digest option just doesn't cut it. 40 rules for filtering e-mail seemed a bit sad also.

I'd be interested in any other comment/links not here.
TO REPEAT: On balance, I'd suggest subscription at the discussion level is a great enhancement we really need. How about voting for it? http://tracker.moodle.org/browse/MDL-1626