Notes based on the Acquia Certified Drupal 8 Developer Study Guide and other resources.
For the record, I officially passed my exam on 4/24/2017 at DrupalCon Baltimore.
Feel free to open PRs, adding new info and correcting any mistakes you may find.
It is highly recommended that you also read the Drupal 8 User Guide as much of the site building section (including the missing notes in this repo) covers what is in the user guide.
Update: 4/2/2018: I'm presently studying for both the Front End Specialist and Back End Specialist exams and have started updating the documentation to go a bit more in depth on certain topics. The material is all still relevant for the general developer exam, but may be a little more detailed than you need. Either way, I feel like it's important information to know regardless of whether or not it is asked on the certification exam.
Update: 4/9/2018: I've successfully completed both the front end and back end exams. As to avoid giving anything specific away, I will say this study guide does have pretty decent coverage for both, but there are a few things missing.
On the front end, it's a bit important to familiarize yourself with some of the newer aspects of HTML5 if you came from a XHTML background. I'll also say just knowing about the breakpoint module isn't enough from a performance perspective. Also, it helps to know some of the more advanced features in Twig, even if you don't use them every day. I highly recommend reading The Drupal 8 Theming Guide. It is a rather helpful resource and definitely proved to be beneficial to me.
On the backend, the best thing you can do is just build a module or two. We could add all kinds of things to the study guide, but nothing beats actual experience here. I'd also emphasize knowing the differences between plugins and services, and different ways you might validate and sanitize user input in a variety of contexts.
Disclaimer: This is not officially related to the Acquia certification process. These notes are just based on what I thought was important while I studied. Your results may vary.