Their inclusion of the WebKit release that ships with Safari 4.x is enough to grin about (which introduced SquirrelFish Extreme and a native JSON object, so clients do not have to implement JSON parsing in AS3 or pass eval'd objects back through the security sandbox).
Beyond that, support for CSS transitions, animations, and canvas are welcome as well. I've not had a need to play with multitouch or the native process API, but they're great to have baked in.
I'm particularly looking forward to the Android SDK. It's a shame the AIR packager for iPhone is no longer maintained, but that's water under the bridge. It'll be nice to have a framework like this which allows for much easier UI skinning and definition than the stock SDK. PhoneGap et al are great - don't get me wrong - but it's nice to have a choice of platforms.
AIR is far from perfect, but it's a solid runtime to work in if you're building an app that has to be cross-platform, is media or network-intensive, and should behave more like a native app than something wrapped in a browser. Kudos to the AIR team on a good release.