Hi David — my name is Ned, and I’m the product manager on this feature. Answers to a few of your questions:
How are folders rendered in the UI?
Folders are an organizational construct, so they will not render anything in the editor UI. Instead, they will expand/collapse in the content tree and will show up in views like All Content, Search, etc. The experience should feel similar to folders on Mac OS. Folders are purely additive, meaning parent pages will still be available if you prefer to add content to your containers.
Can a folder be thought of as just like a page but without any content?
Yes, precisely!
Can a folder have attachments?
Attachment is kind of a term of art so I’ll disambiguate here for clarity. You cannot upload e.g. PDFs to a folder in Confluence. Folders can only parent object that you can create in Confluence, meaning they can contain pages, whiteboards, databases, smart links, or other folders.
What can go in a folder?
Answered above. Folders can only parent objects that can be created in Confluence.
Is Confluence now competing with Sharepoint to have huge piles of documents in random nested folders?
Confluence is not a file storage product. We’re adding folders purely to provide our users with another way to organize their content,