Accessibility: does it provide primitives that allow implementers to improve functionality with tools like assistive technologies, physical limitations, etc?
Internationalization: does it have mechanisms for integrating language preferences? e.g. how hard would it be to switch between RTL and LTR languages?
Portability: if it supports cross-platform, how does it approach it? Does it try to maintain visual consistency or does it prefer to adhere to platform-specific conventions?