The core problem really stems from the idea that OO models encapsulate data and behaviors. Behaviors mean code execution - so, anything that will deserialize objects is giving the person who serialized them the ability to control the execution flow. If this is a listener on the network, than things are really bad :-)
So, it's great that a set of gadgets have been removed, it's neat to see the application of resources to make that happen. I have to agree with Ben, that any system that relies on object serialization from untrusted sources (in any language) is still vulnerable, it just might require a more specific gadget chain. Too many vendors have fixed their products by just updating the library and not removing the dependency on dangerous object deserailization.