On another thread I asked why an application would not work in Leopard and the answer I got was "Some applications look at the version number and won't run if it's not an approved version. All the developers have to do is recompile and their app will work". That's nonsense. I don't understand why so many applications are not working "out of the box" with 10.5. As an experienced developer in windows and unix, but a newcomer to OS X this is a little disconcerting. What are the major causes of incompatibility with Leopard?