1. If then conditions. For example, have particles per second be set to 0 at a certain time, or position of another layer; could also have the particles move in the opposite direction of another layer, such as a text layer.
2. The ability to add sliders to add to an expression. For example, have an object move around like it is a bit crazy. Then with the slider controls I can keyframe the values that go into the random expression.
For example: wiggle(Freq,Amp)
Then have have slider for the frequency and amplitude (amount). I could keyframe the value or add a random expression to make things more interesting. I could have a minimum and maximum frequency, same goes for amplitude. The random expression would then determine what the amplitude and frequency is at any given moment.
Hope this clarify things. I do like Motion, guess I do not have a good understanding of parameters.