Turns out our power users were using watches in Forums, not for the email, but for the indicators in the UI. They would put watches on threads they're investigating, and then could go to the forum thread list and see little icons next to the ones they're watching.
I think I could hook up something similar, either on Your Stuff > Email Notifications or on the Community Discussion tabs itself, although not sure if there's a performance impact on the latter.
But if/when you implement a Favorites feature, please take into account this use case and
recognize that I might favorite hundreds of threads that I'm interested in or working on
include graphical indicators when I'm looking at an object I've favorited.
This is great feedback! Thanks for pointing out that we need to keep this stuff in mind when we work on a more general favorites feature.