When OS X was new, Apple was still under the assumption that Java on the desktop was important, and they built an in-house Java with full Aqua support. It was still _terrible_! All the Aqua-specific affordances like animation or shadows were janky or absent. Sizing and positioning always felt weird because the application was written assuming Windows-shaped controls.

Basically, cross-platform GUI only looks good on the platform that it was originally designed for. Unless the other platforms make zero interesting choices, they will always look worse.