Pros and Cons of Platter Mass

The best direct drive motors do not require as much mass as the best belt drives.

No.
If you look at the Technics SP10 series - the most powerful SP10mk3 has a mucher higher platter mass (25lb ) relative to the weeker motored mk2.
Similarly with the Kenwood L07D - highish platter mass was oart of the design - the argument was high inertia, resistance to stylus drag.

Finally the most powerful direct drive motor built by Technics for lathes - the SP02 - dwarfs the motor in the SP10mk3 - but it was specified to be used in the Neumann lathe with an added 70lb flywheel.

If you build a platter out of weetbix, it will still sound like weetbix irrespective of drive topology.

It is becoming more common for manufacturers to use a magnetic thrust mechanism. No ball and no thrust pad.

And it is a stupid idea - you cannot accurately measure the groove when the platter is not grounded or referenced to a fixed point relative to the tonearm.

Same as air bearings, platter is not grounded, and it is not possible to accurately measure the groove.

Depends on the design my platter which is all copper weighs 80 pounds on TW AC3 turntable sounds great.

My ears tell me to respectfully disagree with dover about magnetically suspended platters.  My Clearaudio Performance DC Wood tt has a magnetic suspension for the platter, and I feel that it imparts a remarkably quite effortless sounding accurate reading of the information in the grooves.

Looking at things from a different angle what "quality" tables
used a rather light weight platter and did they have a particular 
reason.

One example would be the Victor TT-101  of which a few here 
own. 

Playing the devils advocate if a heavy platter is not rotating 
at the correct speed is it then more difficult to manage/correct?