Hi,
1. The ability to add/subtract objects from a rigidset so that you don't have to delete a set and create a new one just so that you can add a piece you forgot to include the first time around.
2. The ability to stagger the animations on different parts so that all the animations don't start at exactly the same time. You could have one animation start after another animation begins rather than at the same time. You could simulate more complex assemblies that way.
I got the thread title wrong, can't edit it out; sorry.