From chatting with various professional wood workers I've come up with two basic considerations for motor selection to keep in mind. I'm sure there are many others but these two seem to stand out the most.
1. The heavier the motor you can use the better (i.e. a 1hp motor would be better than a 1/2hp motor.) The concept here is that you want to make sure that your sander has the power to complete the task at hand so that you are not facing premature motor burnout.
2. Multiple speed control. Different sanding grits sand better at different RPMs. I never really gave a lot of thought to this, until I started looking at high end sanders. I noticed that almost all of them have some form or speed control.