Well, and to offer the case against:
One of our principles is that board divisions should always help make it easier to know where to post, not make it more confusing. I'm against separating Shopping & Cooking because it enforces a sharp dividing line between two topics I don't find it easy to divide, since so much of my cooking is inspired by my shopping and I assume I am not alone (see things like the Copper river salmon thread).
But for me the real problem here is taking a board which is fairly lively and dividing it into two possibly more isolated and quiet boards. Right now, whether I'm shopping, cooking or both, I check it regularly just to see what's happening. But I might be more likely to skip one or the other if it was suddenly half as likely to interest me at that moment. The risk of quieting down one or the other of the topics through isolation seems much greater to me than the benefits of dividing stuff up.
When we were first talking about ways to divvy up the boards I was ready to make lots of micro-boards, but as I read about board management online and as I saw our board in action I came to realize that the best situation is one in which a board is full almost to bursting-- the best organizing system is always the human brain reading and sorting for itself, a board's scheme should do the minimum of basic sorting but leave things broad enough that there is plenty of activity at all times.
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To which Vital Info added some points about how it kind of doesn't matter if a board gets big because there are other more efficient ways to use the site besides visiting it board by board, anyway:
First, when you click on "view new posts", you get all new posts, not just new posts from some board. Second, there is this color/shading thing on the main page that shows when a board has new action. Third, if necessary or for time sensative posts, someone (any mod?) can make it a "sticky" so that it stays on top.
Of course, the argument that board divisions don't matter kind of cuts both ways...
Also:
If you use the search function, you can designate that you want to search in Shopping & Cooking. That's efficient even if S&C has a billion posts, especially thanks to Ann Fisher's very helpful search primer. (That was nice, be sure to thank her.) But if it's two boards, I have to decide if leaf lard pie crust is more likely to be in Shopping (where to find leaf lard) or Cooking (how to cook with it), and search accordingly. Twice the searches, half the efficiency.