The main changes are found when visiting app/music/book/movie/magazine titles with a redesign that follows Google's Material Design guidelines. In a nutshell, this means less clutter and more of what you want to see, with a big focus on content.
What we're interested in is the improvements made to recommendations in the partly visually-refreshed Play Store. On versions previous to this one, the +1 button would be near the top of the page, underneath the content's title and screenshots. And on 4.9, the button has been made larger but has moved to below the review section next to 'Share' which not only lets you share to Google+, but to other apps installed on your device as usual.
|With 4.9, you'll find the +1 button near the bottom of the page|