Google is constantly testing new design elements, but over the past few months they've been testing a change that, while it might seem small on the surface, represents a major philosophical shift. The screenshots in this post were all captured on live SERPs but appear to be tests and have not rolled out permanently. Here's an example of the basic change:
Notice how each result (ads and organic) is wrapped in a container and visually separated on a gray background. These containers are called "cards" in Google's vernacular, and they're important, but we'll get to that. Why should we care about a few borders and a background?
Shift #1: Mobile-first design
We've known for over two years that Google was shifting to a mobile-first design philosophy. Earlier this year, Google removed ads from the right-hand column. While this change was partially due to performance, I believe that a big part of it was standardizing the ad environment across platforms (mobile, tablet, desktop, etc.). What's not obvious from the test above is that this card-based design is more than just boxes and backgrounds. Google is testing a serious move toward single-column SERPs. The removal of ads from the right-hand column was only the beginning.
Here's a SERP screenshot for "polar bear" in Google's current desktop design: