4. I have worked with haxe quite a bit and I haven't seen this; after a while you create enough libs for yourself to avoid this. Or maybe i'm missing the point here; what kind of host-specific features are you talking about? Maybe you are creating totally different software from what I am making :)
Networking, drawing to the screen, video and audio, click events, etc. I think that's most of what most apps are doing. If you're doing something numeric (like a simulation or a computer algebra system) my previous statement doesn't apply.
But these things are encapsulated; if you work with haxe a lot, you'll have libs which, besides maybe a config setting per platform, are code compatible...
