Interesting thoughts. My experience has been mainly with 80s plus machines, and I just haven't played many 60s machines.

The idea of playability is important not just for pinball machines but for all sorts of games including computer games. The classics all have great playability.

To me the ultimate test of playability is the 'just one more go' test. If the pinball machine (or game) makes you want to have 'just one more go' you know it is good in the playability area.