Very simple and easy to use, I would suggest 2 things though:
1. Remove the ajax interface, so it's possible to use the reference offline, also it means you can tell the client to cache the entire thing (as it is static).
2. Abstract the javascript code into a framework that lets you just pass in a heirarchical js array of definitions.
In fact, it's a perfect idea for a webapp, you provide the interface, and people can plug in their own 'definition' list for whatever they are working on. So if a person is developing an app, he can document the usage of it in a simple js array and plug it in to get a snazzy interface.
