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