I wonder, what do you get over exporting a file system over webdav, if you only do "document storage" of json documents? After all filesystems are made for storing a hierarchy of files... some indexes for searching? More convenient rest api?
The API is much more convenient, and last time I checked, WebDAV didn't provide the same wealth of query options or multi-node replication. Plus, I can get solid commercial support for MongoDB, which matters a lot if you're using it to power a business.
Multi-node replication for webdav would be done at the file system level (eg: gfs or whatnot). By query options, to you mean some kind of join, for example? Obviously anything beyond plain id-based get/set (by file path/url) isn't possible with "just" webdav. But if you already limit yourself to looking at single documents (ie: json-files) -- you could always fetch doc1, find the id/path of doc2 in doc1 and fetch that. I'm not saying this sounds like a sane db strategy, I'm just not convinced doing the same thing, with a little easier api sounds like a sane strategy either?