Direct drive vs belt vs rim vs idler arm

"These things all come down to the quality of the implementation, whatever direction is chosen."

Exactly. :)

First, belt drive tt's in general have high mass platters.

Tony, some belt drive decks have high mass platters, but MOST? Some certainly do, but I would venture to say that most belt drive decks on the market, both new and used, do not have or were not designed with platters that I would characterize as high mass platter.

But I guess it depends on what you consider to be "high mass".

I'd say that high mass would be something like 100 times greater than the mass of the record. That would be about 15 kg.
This discussion got me in the mood to check my platter speed again. After playing several records the tt was warmed up and I know from measurements before that warmup is critical. I had a small bit of drift (about 0.5Hz out of 3150 Hz) after setting the speed several months ago. I dialed speed back in and had some good results: -0.01%/+0.02% Wow&Flutter and the speed plot over time shows a sine wave with a 5 second period. I believe the +/- speed variation over 5 seconds is the control system holding speed about the setpoint (At any rate I feel these values are well below audible detection and are an order of magnitude better than the records themselves.) This app is a very good objective measurement tool, I think, for dialing in speed and checking to see if your tt is operating within the manufacturer's specifications. No more, no less.

Here's an interesting test for those of you who have belt drives, especially those with rubber belts like the VPI's. Place your finger lightly on the belt where it leaves the motor capstan and is being pushed to the platter. Feel that vibrational energy that's being transmitted to the platter? You don;t feel it as much on the other side of the belt, where it is being pulled off the platter to the capstan.

A simple solution is to place something fairly heavy and smooth up against the belt between the capstan and the platter to help dampen and absorb that vibration.

I don;t feel any of that with my silk thread, and I would bet there is little to feel also with mylar tape.

Here's an interesting test for those of you who
have belt drives, especially those with rubber belts like the VPI's.

Right, but this is based on VPI's ultra low quality. But you find that very
often with the stuff from other Manufacturers, too. Ever checked a Raven
belt? The word "Precision" you find mainly in their advertising
ads.
Try that with a Basis belt :-)
Or try a Timeline, it is a disaster for the majority of Manufacturers, they
aren't able to offer a proper motor, even when you can buy 3 of them.
Based on that inferior Design solutions I agree that a direct Drive can
give better results. But this is not based on its Design, it is based on the
low quality from the others even when they are very expensive. The
majority of owners believe that the offered product (whatever it is,
Tonearm, Cartridge, Speaker...) is simply perfect form the Design, the
Manufacturers and his cohorts are directly sent from God to bring joy to
everyone and it can be made better, maybe only with more money, but
they don't even think about the correctness of the Design itself. Or what
they really get for the money. Most learn by spending money, but even
that does not educate generally.