Communities are not comment sections
A thread under a product is a reaction to it. A room is a place people return to. Designing the second one taught us why the first was never going to be enough.
We built the comment section first, because it was the obvious thing and it took a week. Every product page got a thread. People posted in them, the threads were pleasant, and almost nobody came back to one twice.
A thread under a product is a reaction to that product. It is shaped by the thing above it, it ends when attention moves on, and it has no reason to exist once the reader finishes the book. Everything about that structure says leave.
A room says the opposite. It has a name that is not a product name, people who are recognisably there on a second visit, and a conversation that was happening before you arrived. The subject can move. Someone can join because of one book and stay for an argument about something adjacent.
The mechanics that make the difference are unglamorous. A room needs a way to be discovered by someone who is not already reading a specific product. It needs the same people to be visible across visits, which means profiles that persist rather than usernames attached to a comment. And it needs somewhere for a conversation to go when it stops being about the thing that started it.
We kept product threads, because a question about a specific chapter belongs under that chapter. But they now link outward to the room rather than trying to become one, which is the honest description of what they are: a place to react, next to a place to stay.