Conservative part here :). What's good about polyglot programming? Abstracting JavaScript with Objective-J? Or even abstracting HTML with JavaScript? I do not understand these tendencies to bring desktop software development practices to the web. It is different environment. You will never get to level of precision when hacking prebuilt widgets.

On the other hand I think that if the front-end is written in HAML instead of HTML, Coffee Script or Objective-J instead of JavaScript will not make application any better. It's often good usability and backend services which matter.