- Targeting iOS, Android, PC and Mac OS X with the same code base, with ANEs to target platform specific functionality as required. There are off the shelf ANEs for almost anything, but you can write your own (or get someone like me to do so) if there's something else you really need.
- Choice of IDE. I use FlashDevelop (which is free, and fantastic) and do everything in code, you can go the Adobe route with Animate CC if you want a graphical IDE.
- Being able to do nearly everything from a PC, including building for iOS. For example, you can build and debug directly on an iOS device - yes, debugging on an iPhone from a PC. You still need a Mac to build for OS X, and for submitting to the app store. I use Xamarin as well and in comparison the build process for iOS is a long slow joke.
- Performance is excellent - you have to write decent code of course! You can run multiple threads, although the implementation is a little awkward at first.
- Regular updates of the AIR runtime.
- As others have said, Adobe's promotion (or lack thereof) of AIR is less than great. I think the (inaccurate) association with Flash is still a problem. Many reviews of cross-platform development tools ignore AIR altogether (while espousing the virtues of less capable tools).
- The general lack of recognition of AIR from the developer community, which may in the long run be self-fulfilling. I think many developers are unaware of AIR's capabilities, or think it's Flash, or are put off by the Flash association. For example, some people seem to think that Apple won't accept apps written with AIR which is completely untrue.Having said all that there is still an enthusiastic (but comparatively small) developer community.
If you're a programmer, use FlashDevelop.
Many cross platform multimedia training tools, health & safety inductions, general business tools, in-house radio streaming, sales presentation tools, interactive artworks, music apps, some small games. As a single developer creating these from a single code base with the least fuss is important.