I'm gonna' go way out onna' limb here and totally speculate....
1. Quality of the bearings holding the shaft.
2. Quality of the magnetic to electrical to physical match of the motor system. Meaning, just how close did this design get to the exact correct diameter of copper (or silver, or super conductor) wire wound onto an armature of what precision of physical dimension, suspended in what Gaussian flux of magnet field, polarized in which way?
3. How good is the amplification and feedback mechanism driving the unit (WOW! now that's a whole thread on its own.).
That leaves out a whole bunch of other important stuff though...
At the end of the day, it's purely power over mass. How much power can you get into the system before it burns up to control the mass in motion as fast and accurately as possible.
But, I just made all that up. So don't rely on it.
James.