Yoast SEO 3.1 partially rolls back the snippet editor design, after 3.0 feedback made it clear that users preferred the old way. The new editor is much improved from 3.0, but they also include the real time feedback feature the implemented in 3.0 — so you now see a little progress bar that highlights green when your snippet meta is in good shape.
The other big feature of note (and the one you may get questions on) is the primary category capability that introduces a new UI element within the category metabox. If the user has more than one category selected, then one is marked “primary” with the ability to promote the second category. So if you see that on a site and don’t know where it’s coming from, it’s Yoast SEO. The feature is primarily helpful for determining which category would go in a breadcrumb trail.