Automatically subscribing people to "Support" and "News and Announcements" sounds good to me, but for "Development & Design" I am reluctant: as a user signing up to the forum to ask a question in the Support category, I probably don't want to get notifications for everything that's happening in the internals of the project.
I think it would make sense to reduce the number of top-level categories as you suggested here: Adding a Design category - #15 by antonin_d