As a former Flex-Developer I would really appreciate MXML support. I would purely use it for a quicker setup of basic screens and layouts. For more complex stuff I would rely on AS, but it is always so painful/repetitive to develop simple menus or screens in AS3, so MXML would be awesome here. Less code and better readability.
I remember Data-Binding always being a bottle-neck in Flex. I'm curious to see how it can be optimised using Starlings Event system and Feathers invalidation methods.
The only bummer for me is the drop of state-support. I tend to build my components with states, showing different sub-components when needed. So a good state-system with the possibility to lazy-instatiate the components when necessary is actually quite handy.
But it is not a killer-feature or should be the main priority. That's something to work out once the feature is stable. Until then, we can anyhow build such thing in AS3 in addition, as We've done till now.
Great work Josh! Looking forward giving it a try.