How are sub-categories created? Are these manually assigned by Cake.co admins?

Good question.

Those are what we call topics. Anyone can create a new topic when starting a conversation (if you type the name of a topic that doesn't yet exist, then it'll be created). Cake admins review newly created topics and will categorize and sometimes edit, merge, or delete them as necessary to ensure that they're high quality.

We didn't call them "sub-categories" because topics are pretty free-form and can belong to more than one category, so it's not a strict hierarchy.

So, I see they can be "followed", but doesn't this get old after a while, I mean with no site wide search, one needs to know beforehand what existing topics to follow or conversations to drill down into. Or how is the best method to discover new stuff? On other sites ;-) once I "like" a post, I am automagically subscribed to the conversation.

Site-wide search just went live today.

In addition to searching for topics, you can also read the "All" feed to see conversations in all topics, which can help you discover interesting new conversations in topics you aren't following yet.