* Pervasively typed with TypeScript, written in TypeScript, completely first-class. (It is very easy to consume TypeScript base code from JavaScript, but tacking types onto something written without them almost always yields a much lesser experience.)

* Output standards-compliant web components by default. (Perhaps with room to also output some other variation of component, if there is some API aspect where that helps.)

(Distraction for another day: I think the future of SPA development will be web components at the widget/component level, while application frameworks start evolving upward to provide more features around building applications - because there is less for them to do around building components.)