Looking back at the last year or so working with Tiger, I don't think memory management was my biggest bugaboo, not even with threads. The hard stuff nearly always comes down either to figure out how to make the framework do its thing, or how to hack it to do what all the other apps do, but Cocoa doesn't do out of the box. The big one was outline views / tree controllers -- completely half-baked in Tiger, requiring a major hack into Apple internals, which will need to be revised for Leopard. Core Data, a wonderful feature with very little documentation ;-) And all those little things, like table views where adding a new object should cause that object to be selected and its text put into edit mode, and then selecting the object again when return is hit... The AppKit still needs lots of work to factor into the core classes lots of behavior that 95% of apps need and users expect, and that "just works" in all those Apple apps.