This may have already been discussed but in my 13 years of internet usage, it is pretty common to get email notifications for new activity in any thread I have participated in, by default. I noticed this is off by default and since this is my second post it may have taken me a while to realize it wasn't turned on by default, then a few weeks would have gone by and I would have assumed nobody replied to the discussion I just created about Flattr
and I may not have come back for quite some time because I wasn't prompted to take action.
So, my suggestion is to enable forum activity notification by default since I believe it will greatly increase participation. This is similar to what Flattr does just in a different way. It prompts action. Action drives movements. Movements make waves.
P.S. I just was about to submit this thread and almost forgot to check the "Notify me when a reply is posted" box at the bottom because I have not yet switched it in my account settings. #userexperience