There were a couple of points Tully made that I was a little confused about so I PM’d him to avoid spamming everyone, but I’ll try to summarize quickly [quotes are from Tully]:
Unfortunately the default is either subscribed to all, or subscribed to nothing. For backwards compatibility we’ve selected subscribe to all to make it more like a mailing list . Then the user can downselect.
I didn’t realize this. I thought I was just getting emails because of the feature where you automatically track a topic if you’ve spent a few minutes reading it. It’s a little confusing because my profile also doesn’t say that I’m subscribed to any categories 
Unfortunately, you can’t unsubscribe from email updates by tag, only by sub-/category (as far as I’m aware). As the default is subscribed to all, it might be better to use sub-categories for projects that warrant it, underneath the Projects category. This way, anyone that wants to unsubscribe from a few projects in particular can do this without unsubscribing from the entire Projects category.
It’s a little extra overhead to ask to create a subcategory but that’s probably not unreasonable. We just need to setup a mechanism for proposing, ratifying, and creating these that’s easy to follow and for the moderators to execute.
I’m happy to go with Tully’s suggestion of following the model of the SIGs, with somebody suggesting it and others +1’ing. It could be that a project’s need for its own sub-category will be self-evident from how busy their original thread is? Personally, I’m inclined to vote for Martin’s suggestion of just getting on with it and figuring out the process the first time we need it 