This is probably not quite the place to converse on this, but I'll make a quick reply.
Subscription at the discussion (Thread) level has a hack found here. http://tracker.moodle.org/browse/MDL-1626 The diagram shows how suprisingly complex this is. It also has some performance implications.
There are changes to the forum module that may come in Moodle 2.1, and an (Unfinished) outline of some of the issues is on a page in Moodle docs.
The OU's ForumNG has this capacity already. This is the OU's in house developed forum theyh ope m ay be included in 2.1. Some discussion on this (including links to a number of past discussion threads) in the forums is here.
I agree with you Gerry: Thread level subscriptions here would be great. It is my opinion it would improve engagement for many people.
-Derek
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