As a web developer, I like JSON because there is a straightforward mapping to JavaScript. This makes it very easy to process the data (iterate through it, read values from it, use it to configure objects).

With something like XML, processing the data requires the DOM (which leads tedious-to-write verbose code, and which has some browser compatibility issues) or an extra dependency on a more high-level API (since XPath and CSS selectors are either not available on many browser platforms or again suffer from browser compatibility issues)

As for the schemalessness of XML vs JSON, I don’t think this is a problem in practice for most browser-based web applications. I haven’t seen many cases where people actually use XML validation, so nothing is lost in that respect moving to JSON. If you’d want to you’re pretty much stuck with DTD’s (that is, if you want to have a cross-browser compatible solution). XML Schema sadly isn’t really well suppported in browsers. In addition, if you really want to have JSON schemas, you can, with the JSON-Schema emerging standard (http://tools.ietf.org/html/draft-zyp-json-schema-03)

Here is my post: http://www.jroller.com/dmdevito/entry/thinking_about_nosql_databases_classification

@Mark,

I’m struggling for terminology here. Replication, sharding, synchronization, scale-out, reconciliation — nothing is perfect. But it’s all in the area of “A query or update comes into the system, and somebody has to figure out which server(s) to send it to.”