One place where the HeapObjects change seems to cause havoc is in the GraphicsKit. There are several places in GraphicsContext.cpp (eg line 161) where a dynamic_cast to Object is made for the purposes of refcounting. Now that's changed to HeapObject, there are classes that originally derive from Object that are getting mashed (it's dereferencing a null pointer so why it doesn't just crash with an access violation I don't know)

But in any case - these are the classes that I believe are affected. It's probably worth checking the tree at the top of this page to check that I'm not missing anything or some other group of classes that should be heapified:

BasicFill
BasicRectangle
BasicStroke
BezierCurve
Color
Cursor
DataObject
Ellipse
Font
Frame::State
GradientFill
GraphicsContext?
Point
ShadowFill