The problem with Wordpress isn't the choice of data store, and I say that as someone who uses and loves MongoDB. Wordpress itself is relatively slow and ineffic

The problem with Wordpress isn't the choice of data store, and I say that as someone who uses and loves MongoDB. Wordpress itself is relatively slow and inefficient in a number of places. It doesn't come with any appreciable caching built in out of the box. It makes bad assumptions about the amount of data you're going to be dealing with. Various plugins are naive and trash your performance. While it'd be neat from a curiosity standpoint to see Wordpress running on a NoSQL database, it wouldn't magically make Wordpress "web scale". PHP is slow and memory-hungry, and if you don't aggressively cache your pages, you couldn't easily scale Wordpress even if you had a magical infinitely fast data store.