I like the idea of separating the content from the presentation, although working with JSON directly to create the content, to me, is not fun.

Did you play around with just using markdown directly and embedding html tags in that in a similar way to format the page? I can't imagine myself intentionally writing web content using JSON. I'd probably have some other format up front that would be converted to JSON, but at that point I may as well just write some static page generator to create the html.