I think this list should include:
 - 'long_name'
 - 'valid_max', 'valid_min', 'valid_range'
 - 'scale_factor'
 - 'missing_value'
 - 'sample_dimension'
 - instance_dimension
 - flag_masks, flag_meanings, flag_values
 - grid_mapping
 - _FillValue
 - add_offset


( note, coordinates, cell_measures, standard_name, units, ancillary_variables are handled as references by Iris, so should never  be in the attributes dictionary as keys)