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.