1) Its fast, memory management isn't an issue for developers like it is on Android or even iOS.
2) Its smooth and multitasks well. QNX does deliver here big time.
3) Its solid -- during the Alpha and Beta stages, I never had a crash -- this is, looking back, astounding.
4) Native development is not as demanding as Android or iOS. The dev ecosystem guides you well to just the right balance between native UI elements and customization and you can do it in a language you likely already have knowledge of. (Java, QT, HTML5, C++)
Overall -- it was a pleasure to build and launch apps on BB10 -- which is more than I have to say for other ecosystems. I'd describe it as the the ease of developing the display layer for a Windows Phone app mixed with the flexibility and depth of an Android app and the power of iPhone hardware.
Background: I'm a hardcore iPhone user who is in the mobile industry and regularly uses hundreds of mobile devices.