I tried this and added a log statement to see what had changed.  I went to a Problem Task and update about 3 fields, but the only thing that was logged was the sys_updated_on field.

(function executeRule(current, previous /*null when async*/) {

    var changeFields;
    var gr = new GlideRecord('problem');
    gr.addQuery('sys_id', current.problem);
    gr.query();
    if (gr.next()) {
	for (var x in current){
	    if (current[x] != previous[x]) {
		gs.log('Field: ' + x);
		changedFields += 'Field ' + x + ' has changed on record ' + current.number + ' from ' + previous[x] + ' to ' + current[x] + '\n';
            }
        }
	gr.work_notes = changedFields;
	gr.update();
    }
})(current, previous);