Pros and Cons of Platter Mass

@lewm , you need to add motor torque to that list.

I think as everything in life there has  be a balance and it depends on what type of drive is being used along with it's performance. The best direct drive motors do not require as much mass as the best belt drives.
Mass does not protect you from rumble. It might lower the frequency a little but you are also adding more thrust to the bearing, increasing noise and wear. 

How much mass is required? Whatever it takes to get the best performance and no more.

What confuses me the most is how a topic sat dormant and was revived 19 years later. I had to check to see if the OP was still with us and certainly glad he is. 
My TTW table’s plater separates into two parts for convenience, but the total weight is 70lbs. The copper weight and periphery ring add more and I’d love to know how much the the flywheel effect adds since the TTW periphery ring has brass weights.
I can say going from a less massive table to the TTW added dynamics and bandwidth to the sound. Also I feel the mass allows vibration from the stylus in the groove to sync, and the center weight and periphery ring holding the LP down essentially make the LP weigh 70lbs lowers distortion tremendously.

It is becoming more common for manufacturers to use a magnetic thrust mechanism. No ball and no thrust pad. Spindle wear is much less of a problem because of the greater surface area involved. Here, lubrication is quite effective if the bearing is designed properly. My subwoofers tell me there is most definitely less rumble with magnetic thrust bearings, at least the one I am using. 

Magnetic thrust bearings also allow more platter mass without increased wear. The same is true of real air bearing turntables. Frank Kuzma redesigned his big table with an air bearing and increased the mass of an already huge platter by about 1/3rd. I wonder if all that air rushing around increases noise. What if it blows on the tonearm?

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.