From a developer's point of view, on one hand, I like this idea. Just about any development environment is better than Android:
1. Callbacks are horrendous. I hate inner classes, they're ugly and unwieldy. I'd honestly rather something like AWT's (Yes, that 90s tech) callbacks over inner classes.
Just have a main function with a large switch. It's much easier to reason that then the current mess-up of trying to figure out your app's lifecycle.
2. Inter-thread communication is horrendous. Android is Java based, so can I pass classes?
No.
Only Strings.
And even that is horrendously ugly.
3. It's a Frankenstein model. It's a Desktop OS like process/file model, but not everything is a file. Now they suggest opening using the new file-picker.
But in many ways files are easier to use.
4. They add new libraries (Material Design, for example) which no one will ever use, because everyone needs to use appcompat. And even in five years, when Lollipop will be like Android 2.2 is now, everyone will still have to use appcompat to get Android Z libraries on Android O, or something. And by the way, now most apps rely on appcompat-7 which relies on appcompat-4.
5. Many classes are broken - Take TextView. You can style it with HTML, but only a small subset (so you can't use it as a WebView). So it's not meant as a "web browser" replacement. But it can parse broken HTML. Wow, so developed.
But you can't style a link color, or change underline color.
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But on the other hand, I'd find it horribly annoying (and I think that many others would feel the same way) having to rewrite everything from scratch into Dart or whatever.