Late to the party on this one, but I thought I'd add my two cents as a new user. Removing the ability to follow by tags narrows the opportunities for new users to get discovered. The "unshared" page helps balance that some, but that throws more weight on the adventurousness of older users. If they don't go out of their way to look at the unshared page, then it's that much harder for the submissions of new users to get noticed. If you want to stick with the following users but not tags, may I suggest providing some mechanism for inserting a few unshared posts a little more assertively in or around each user's feed?
If you go in to "controls" you can up your "external posts" (posts from people you don't follow) to "many" and that will do what it is you are asking. I do think that perhaps we can make the external posts more differentiated looking. What do you think mk?