The recently revamped notifications arearecently revamped notifications area is the perfect place for these. But I hate the idea of separate opt-in / opt-out.
Folks have various uses for favorites, but they mostly fall into two categories:
- Stuff they want to keep an eye on for some reason
- Stuff they want to be able to find again quickly
Of those, #2 has never been particularly well-served. There's no search, no sharing - the only real benefit is that the list is attached to your account, so you can retrieve them from any device. Of course, this functionality is offered by a dizzying array of dedicated services, and increasingly build directly into web browsers... Along with search, sharing, etc. Trying to duplicate that on the site makes about as much sense as building a spell-checker into the editor.
But #1 actually was effective at one time. Yeah, some folks complained about it. But a lot of that was due to lumping those notifications in with more direct responses, those "specifically targeted" events that you mention. Avoid teasing us with "you've got mail!" every time something happens to a post we're passively interested in, and I expect a lot of those complaints will disappear. For those that don't, well... Opting out is as easy as clicking that little star again.
Also... This is the last little feature I liked in Quora that hasn't yet been implemented.