The divider between the pinned posts and the new posts was the big thing for me also.
However, while we're on the topic of forum changes, something I'd noticed recently, possibly not related to today's change, is that in the past when I had completely read a forum thread, the title was unhighlighted and the box to the left of the title was unchecked. Now that still happens, but only for forum threads that have at least one reply. If there's only the forum header, the title remains highlighted and the box checked as if I hadn't read it at all.