[danik.js]

Итого, все умещается в 15 строчек

<form name="form_1" action="">
    Combobox 1: Ungency
 <select class="element select medium" id="elem_1" name="elem_1">
 <option value="" selected="selected"></option>
 <option value="1">System does not work</option>
 <option value="2">Critical bugs</option>
 <option value="3">Small problem</option>
 </select>
     
 <br />
  Combobox 2: Impact  
 <select class="element select medium" id="elem_2" name="elem_2">
 <option value="" selected="selected"></option>
 <option value="1">all Users</option>
 <option value="2">lot of user</option>
 <option value="3">Individual user</option>
 </select>
    
  <br />
  Textfield: Priorization
 <input id="elem_3"  name="elem_3" class="element select medium" type="text" value="" readonly />
<script>
var form = document.form_1;
form.onchange = function() {
  var ungency = form.elem_1.value|0;
  var impact = form.elem_2.value|0;
  if (!ungency || !impact)
    return;
  var priority = Math.min(ungency + impact - 1, 4);
  form.elem_3.value = "Prio " + priority;
};
form.elem_1.onchange = function() {
  if (!this.value) {
    form.elem_2.value = '';
    form.elem_3.value = '';
  }
};
</script>