Feature request: Opt-in to watching categories instead of opt-out

[henrywright]

Currently watching categories is opt-out. On forums having lots of new topics, this results in a huge number of items in example.com/unread. I think the unread page would be far more useful if watching categories was opt-in. That way /unread would contain only the items a user is interested in from the start and they won't have to go ignore a bunch of categories in order for /unread to become useful.