Relative schedules, roles -> users
In my view these two together are the "killer app" of project management software.
Firstly, relative schedules allow you to define "D-Day" or "Zero Week" and define all other due dates relative to them. For example, you know that the party happens on October 1st, and so "Organise Venue for Party" really needs to be done about three weeks before the party date. So you set it accordingly, relative to 1st October. Then when the party is postponed, everything else moves too.
Secondly, the roles feature in Redmine is great! But you need to be able to assign a group of tasks to a particular role, and more than one group or role to a single user.
Together these would make Redmine an excellent "generic" rather than development oriented project management suite.
Congratulations on an excellent app.