User Registration with REST
This now works in Drupal 8.3-x-dev. This is an example to do it from Angular:

  /*
   * Register new User.
   */
  register: function (email, password) {

    var defer = $q.defer();

    $http({
      method: 'POST',
      url: config.endpoint + '/user/register',
      dataType: 'json',
      data: {
        'name': {
          'value': email,
        },
        'mail': {
          'value': email,
        },
        'pass': {
          'value': password,
        }
      },
    })
      .success(function (data, status, headers, config) {
        defer.resolve(data);
      })
      .error(function (data, status, headers, config) {
        defer.reject(data);
      });

    return defer.promise;
  }


Make sure you enable the resource to enable user registration (I use the REST UI module).



The last step is enabling the permissions on this resource:



Now you are all set!