Currently I can get every email from a forum or perhaps a daily digest. What I would love is to get immediate emails just of threads I have posted on, so I can see when my questions have been answered or when someone I have helped has replied. I believe every other support forum I have used (stackexchange, ubuntu forums, etc.) does this and it helps keep me engaged in community support. Moodle's option of a constant stream of mostly irrelevant emails or daily emails means I am less engaged.
I have always loved that Moodle "eats its own dog food" and runs moodle.org on Moodle, but a global support forum for a huge software project really needs thread subscription, while a classroom can get by with the existing options (or install NGforum).