Lots of little things with various results. The breaking changes list got me through 99% of them, here’s some of the others, and I’ll probably update now and then with more.
1 – Image.Source property. Before, I could use this to find out if I’ve already initialized an image (in a hover-popup in this case). Now, it returns a valid BitmapImage, even if it’s empty. Good, but not what I expected and it failed silently until I realized my previews weren’t showing.
if ( PreviewImage.Source == null )
2 – ActualHeight and ActualWidth seem to be getting set later in the first layout pass than they were before. This led to some issues in setting a ScaleTransform during the first layout pass.