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.
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 do not typically see in traditional style guides, such as instructional FAQ text, web app labels, forms and webinars.
The clever part though is how the instructor communicates the guidance. For each content type, the guide includes the user’s point of view along with how they’re feeling, paired with example text staff can use and tips for writing appropriately:
This is a great way to provide guidance. It’s good too that MailChimp shared their guide in the public domain.