Ditto for Coffeescript. I realise you can't just remove it, but the whole dev community is raving about ES6 - there's a whole of JS devs who could be a lot more interested in Rails.
As for ES6, there is the sprockets-es6 gem. According to their github gem, their goal is to get in es6 support into sprockets 4. This, obviously, is not a perfect solution - but I think this is going in the right direction.
Does it directly impact me now that I choose to ignore it? No. Does it impact a newcomer, particularly one that's into JS? Yes.
It's not perfect but it's how I learned coffeescript.
Other than that, I don't think Rails necessarily pushes either for/against coffeescript. You can either use it or not. Plenty of rails apps have been written in plain 'ol js.
The server-side partial replacement, for example, is wonderful.
1) I hope ActionCable is as seamless to integrate and works as well as Rails features ought to; you can, of course, achieve web sockets right now using Faye or various other implementations, but it's quite complex to get right (though the faye-rails gem is pretty handy) - and realtime functionality is becoming more and more important.
2) I wish they'd just give up on Turbolinks. Maybe some people are using it and love it, but it's always the first thing I turn off when I start a new Rails project, and I know quite a few people who feel the same. For those of us who never thought it was a good idea in the first place, I'm not sure any number of improvements are going to change our minds.