The main improvement mentioned on the update is that BBM has a number of performance improvements. After having played with BBM for a few hours I can't say that I've seen a huge difference in performance. The blue "app loading" circle still appears if you keep BBM in an active frame for a while. Stickers are still taking a little while to load up initially. What is probably improved is the loading time for the BBM smileys.
Whenever we tried to add a smiley to a new BBM chat, there would be a noticeable pause before it would start drawing the smileys. That seems to be much smoother now making chatting that but more friendly.
Photos in Groups
Probably the biggest (yes, I saved the best until last) change is the added feature of being able to add a photo within a group chat now.
The photo will still appear in the Pictures pane of the groups for commenting - but now you can see it in context of the conversation. A huge improvement.
Have you spotted any other new features that BlackBerry are being coy about? Let us know in the comments!