The one thing that I never saw make the leap from BeOS to other operating systems in a meaningful way was the "an application is a thin wrapper around a directory of metadata-enriched files" model that BeOS did.

I think the mail application and the media player were largely built this way because Be needed to build a handful of demonstration applications quickly and cheaply. On the other hand, I remember thinking how nice it was that email and multimedia behaved like the file browser with all its search and filtering features.

iTunes always felt like the exact antithesis of the BeOS model. It was a heavyweight application that had its own interaction model and idiosyncrasies. It might as well have been its own OS by comparison.