Post replies to forum threads through email
It would be very useful if when a user receives an email regarding a new message or reply on a forum, that they could simply 'Reply' to that email to post a response. The advantages would be huge as it would allow for easy centralization of project dialogue, and decrease the barrier for users interacting with Redmine. This is especially important for people who are predominant users of email and dont want to switch windows everytime a forum message pops.
Functionality would be similar to what Google Groups currently accomplishes. As such, for some of the groups I'm currently involved with, this is the make or break deal for using Redmine.