Personally I think this is a great idea. In fact, this is one of the reasons why I like Rails so much – that I don’t have to spend hours/weeks researching and agonising over which technologies to use. It’s like delegating the decision to someone else. Someone smarter, more experienced and knowledgeable. And someone who’s delivered the goods in the past (ref DRY, REST, Convention over Configuration, MVC, testing etc)
Yes I’m a Rails nube, and most of those disagreeing with the decision are not, but I’ve yet to see a good solid argument against it. Most of the reasons seem academic, “Steeper learning curve”, “It’s your choice not mine” – reasons I don’t think are really that big of a deal. Nubes will defer learning the new tech till later, and those who don’t like CoffeeScript or SASS simply don’t have to use them… just as it is now with any of the other defaults that you might not happen to like (such as Test::Unit).