The old guideline that was changed was from:
To the new guideline:
In May, Google introduced a new fetch and render GoogleBot feature and told us they arefully rendering your HTML web pages, just as a user would render them.
To quote Pierre Far from Google via Google+:
Let me be super clear about what this means: By blocking crawling of CSS and JS, you're actively harming the indexing of your pages. It's the easiest SEO you can do today. And don't forget your mobile site either!
The mobile reference is super important as well, read more on that over here.
Forum discussion at Google+.