Why do turntables sound different?

Anvil, With reference to direct-drive turntables, you wrote, "Coupling a motor to the platter, in my estimation is a compromised design."  You are not the first person in your industry to make such a distorted statement, so I am sensitive to it.  In a direct-drive, the platter IS the rotor of a motor that has its stator mounted symmetrically around the spindle.  There is zero possibility for the coupling to introduce noise, except for the noise of the bearing itself.  Bearing noise is a bugaboo common to all types of turntables. Quiet operation is a virtue of direct-drive, not a problem.  DD introduces other possible issues, but not that one.

As to your statement about DD lacking microdynamics, this can happen if the motor causes counter-rotation of the stator, because of the hi-torque forces involved. (Motor drives platter in clockwise direction but its torque exerts an equal force in the counter-clockwise direction. Per Newton's Third Law of Motion.)  If a tiny amount of counter-rotation of the motor or its parts occurs, this results in a perceived speed error by the servo, and the servo exerts a correction.  Thus there can be a constant hunting for speed on a very micro level that is not readily perceived as pitch error but as a loss of detail or a mechanical sound.  This is why DD turntables do best in massive plinths, IMO.  But also, Richard Krebs has addressed the issue for Technics tables with his internal mods that reduce the capacity of the stator to rotate against its own force.

I don't waste my time arguing for one turntable technology uber alles.  I have found what I like, and I plan to stick with it.  Probably the same is true for others participating in this thread who champion other drive technologies.  

Inna et al, Like any top dogs in any field of endeavor, both the Continuum Caliburn and the Rockport Sirius have their detractors, even among belt-drive aficionados.

has2be, In some instances, the design calls for a lubricant that per se creates some drag on rotation of the platter, so the motor has constant small resistance to work against.  This can be devised to improve speed stability. Best example I can think of are the Garrard 301 grease-bearing types.  Friction thus created does not produce noise.  It can also be done magnetically, in some other cases, but you might fairly argue that magnetic drag is not a kind of friction.

I mentally divide up turntables into categories. First we have belt vs idler vs DD. Among belt drives, we have designs that favor massive platter with weak motor (Walker and Notts), massive platter with powerful hi-torque motor, lightweight platter with usually hi-torque motor, or lightweight platter/weak motor. Then you have suspended vs unsuspended, massive plinth vs lightweight plinth. Then you have platter materials to consider. Then you have bearing design and execution, as noted above. Then you have the possibility of magnetic or air levitation of the platter. Then you have direct or indirect application of torque between motor and platter. Then you have platter mats, which can make a huge difference. So, this only covers belt-drive. Which is why it would be most surprising of all if all turntables with pretensions to excellence were to sound the same. Which is why I cannot get my arms around this topic.