Local plugin (Moodle 2) for allowing email subscription to discussions (threads) and not the whole forum.
Discussion subscription can works together with full forum subscription, but the user will receive the same emails two times.
Discussion subscription supports also diggest if the user has enabled this option in his profile.
The plugin is not yet approved but the code is available here:
https://github.com/jleyva/moodle-local_dsubscription/zipball/master
http://docs.moodle.org/22/en/Forum_discussion_subscription_plugin
I will updated this post once the plugin is approved