"Does anyone know what is so special about the big M-S turntables, apart from their obvious build quality, which can after all be matched by several of today's high end belt drive turntables?"
Hi Lew,
In my opinion, the string drive, combined with high mass and speed control, is key to the performance of those turntables. The string is a limited, or more properly described as a calculated, slip. A typical belt drive has inherent belt creep that is difficult to overcome, so it is typically less accurate overall. That means a string drive is easier to control for accuracy, and Micro Seiki made a competent controller for that task. Then, the mass does its job with inertia and resonance control. All that makes for a very good turntable, and the other features are lesser in importance, but because of their precision play an important role.
Caveat: Note my use of "in my opinion" and "typically" in this post. I have heard some exceedingly good belt drives that are more traditional in their construction. The Fairchild 750 is one example, but there are others.