Grooves / Michael, I disagree that hall effect commutation has the effect you say it has, I think the problem is much simpler and was first "outed" by Sansui's engineers 20 years ago. It comes down to Newton's Third Law (you know the one, equal and opposite reactions etc)
The motor control electronics continually vary the drive to the motor to keep the platter speed constant. Any competently designed servo loop does this to an exquisite degree, virtually eliminating speed variation as a concern, but the variation in drive simultaneously torques the motor's mounting in the opposite sense.
By definition in a direct drive this counter torque is transmitted to the turntable chassis whence it proceeds to wreak havoc. How the designer copes with said havoc defines the success of any DD effort and it means that the designer is faced with a very different set of challenges from the designer of a belt or idler drive TT.
Mark Kelly