Here's what I'm taking out of this thread:
* Platonides mentions the case of power-users with tens of scripts loaded via gadgets or user JS with importScript(). * Tisza asks that core onload hooks and other functions be overridable by user JS. * Trevor and Michael both mention i18n as an important consideration which I have not discussed. * Michael wants certain components in the js2 directory to be usable as standalone client-side libraries, which operate without MediaWiki or any other server-side application.
-- Tim Starling