GNOME designer Allan Day posted a very nice article to highlight some of the upcoming changes for GNOME 3.16. Read Allan's article for more screenshots.
Allan credits Florian and Jon for redesigning the new notification feature. In previous versions of GNOME 3, notifications appeared at the bottom of the screen. The challenge that Florian and Jon addressed is missed notifications. Allan explains: "One of the key goals for the notifications redesign was to provide an effective way to review previous notifications. In this way, notifications can be used to find out about what has happened in the past, as well as what is happening right now."
While I haven't reviewed notifications as part of my usability testing, my heuristic evaluation is that the new notifications design will improve the usability around notifications. I believe most users will see the new grouping of notifications with calendar events as making a lot of sense.