Also, the solver along with its effector are now an object that can be selected and moved in the world. This makes it much easier to move an IK animation to a different part of the world without ruining it. In addition, forward kinematics and IK can now be blended together in the same chain, allowing parts of the IK chain to behave with forward kinematics. The IK is easier to use, more stable, and able to withstand abuse without going kablooey. Bones in general are much easier to use because they behave more like objects than like helpers.
Any object can be a bone and any bone can be an object.
This is easier to work with, and it just makes more sense than the way bones were implemented previously. The Skin modifier is more sophisticated. Its new angle deformers allow more specific control over the shape of the skin at specific angles in a joint. And the flex modifier now has dynamics, including springs and collision detection. Dynamics can be used to automatically create secondary animation in a character's appendages.