New FSX features for addon developers:
New animation system which is run independently of "what is animated". Although animations are mostly used for animating models, the animation system could be used to animate other components, say the camera for example.
Support for more than one animation affecting a part at runtime and from within the 3D authoring tools.
Removing the hard-coded relationship between the name of a part and the animation which is associated to a part.
The visibility of a model's part can be procedurally controled through the use of a scripting system, removing built-in hard-coded behaviors for things such as tires on vehicles.
Built-in event system allowing specific events to be triggered from within an animation. For example, this could be used to trigger a certain special effect at a specific keyframe within an animation.
Basic support for Inverse Kinematics (aka IK). Not to go into details, but this allows the procedural animation of an object in order to reach a certain position target in the world. For example, we use this to procedurally animate jetways to the door of an aircraft.
source is here:
http://blogs.msdn.com/sebby1234/archive/2006/04/08/568204.aspx