A lot of the most useful stuff has to be tied to specific implementations, if not in API, then in detail. Most of that stuff would be interfaces to non-javascript libraries.
Or at least extending the javascript specification to include an API that all of the engines can implement. I think this is basically the one thing keeping javascript from entering the mainstream as a general purpose language. As soon as you can reasonably tell people that their server side javascript will run on more than one engine, you'll see a lot more development.
