A lot of our effort goes into various user interface components. The MooTools library gives us a lot of tools and methods to extend them, which we do in the Fx, Native, and core extensions outlined here. But the majority of our time is spent working on UI issues, and we typically only extend the core when we have a UI need that MooTools doesn't solve for already.
We distinguish "Layout" components from "UI" components in that the former is meant to modify content already in the page, while the latter injects new things (like popups) on the fly.