Don't forget E4X, it's here in AS3 as a first class citizen, while every single browser vendors either did not bother to implement it (google) or deprecated it after some times (mozilla).

It was quite nice to use client-side in Flash and AIR for such things as SOAP etc. but it is also quite powerful when used server-side for web scrapping and HTML templating.

I see that every day with Redtamarin

https://github.com/Corsaair/redtamarin

it does only AS3 on the command-line, there is no GUI rendering of any kind, and yet the programming language that is AS3 shine.