> What would it take to make an SSG appeal to a broader set of users?

I don't think the SSG, as they largely exist today, will ever gain widespread usage by mainstream users.

But one thing that could drive greater adoption would be to develop an aesthetically pleasing web front-end that lets users write content using familiar WYSIWYG tools, but save the content to local files instead of a database. From there, the process would be largely the same as it is now: transform this directory of lightly marked up plain text files into raw HTML.

So, to answer your question: Make it more like Wordpress.