I like the approach of listing different content types in the left-hand column. This structures the guide around tasks, not particular points of language. I bet this improved take-up in using the guide. Nice and simple. Clear.
Today you might be writing content for a newsletter. Tomorrow you might have your head down figuring out labels and help text for a web app. Structuring a style guide based around such tasks reduces the effort of navigating a lengthy document, hunting the relevant section.
I also like how MailChimp include voice and tone tips for content types you don’t typically see in traditional style guides – for example, instructional FAQ text, web app labels, forms and webinars.
The really neat part though is how the instructor communicates the guidance. For each content type, there’s a user’s point of view along with how they’re feeling – paired with example text staff can use and tips for writing appropriately.
I think this is a great way to give guidance (and great that MailChimp shared their guide publicly).