Front-end Registration Form just refreshes with no error displayed and user not created
Solved the mystery. :) The issue was that even though my user fields were not required, there was a numeric 'year' field which had min and max values set, and this was throwing it off (and I had removed that field during my last round of testing to keep things simple). Removing those required min and max values allowed the form to be submitted. Thanks for the help Brad!