A unified design across different screen sizes provides a better experience
In working to unify its different platforms, Google is rolling out updates to Google+ on the web and mobile apps to create a more consistent experience across screen sizes. The big focus from the keynote is on the web component, which has taken on a view with multiple "cards" and columns in the same way that you'd expect to see on the Google+ app on a tablet. The design is responsive and resizes based on the size of your screen and webpage to give more or less information. The cards feel more "alive" and move as you scroll along them.
Google is also introducing new features for these web cards, such as auto hashtagging for images you post, related hashtags and posts based on cards you see, and many more. These features are just hitting the web starting today, but we would expect many of the same features to be folded into the mobile apps in their own way soon as well.