A few more thoughts then:
As you mentioned, you get an email notification for subscribed topics/forums, so thinking about it, I don't really see any need to put any features around that in Smartfeed at all. If you want those topics then you'll either get the email, or chances are you'll be subscribed to the whole forum through the feed.
With regard to user status, Normal and Founder should get custom feeds. Anyone else just gets the Guest feed. Ignored or inactive users could simply log out which would make them guests, so trying to hide stuff from them would be a waste of code and coding time. Besides, Inactive doesn't mean they are bad people, usually just that they haven't activated a new account yet.
To the comment by Jim about a generic feed, what I did was set the RSS icon auto-feed thingy to use the URL that is generated when you use the interface as a guest. That way, guests didn't even need to use the smartfeed_url interface at all or even be aware of it's existance. Perhaps though an admin option in Smartfeed so that smartfeed.php with no options given presents an admin defined "default" feed? And the smartfeed_url just redirects to the admin defined smartfeed selection if the person isn't logged on?
BUG! The forum selection thing is still buggy. Hopefully you can follow this example:
1. With all forums ticked, untick "Tavern News", and "The Tavern" also goes unticked. "The Watering Hole" is still ticked though. All good so far.
2. Now tick "The Tavern" on. "Tavern News" and "Watering Hole" are both now ticked. Still all good
3. Now untick "The Tavern". The whole branch is now unticked. Moving along nicely.
4. Lastely, tick "The Watering Hole". Now "The Tavern" (and "All subscribed Forums") become ticked again, even though "Tavern News" is still unticked. So we have a different selection of ticks compared to step 1.
The same complication comes about if you play with the forums that have sub and sub-sub forums.
I would suggest simpler approach. Toggling a parent forum or category simply toggles all children below it to the same value, but setting a child forum makes no changes to the parent selections. Unless you have a "grey filled box" option to indicate the partial selection I think this would still work as a logical method.