Ultimatum,
Вариант...
var json = [{"id":"300","text":"привет"},{"id":"301","text":"reboot"}],
temp = [{"id":"300","text":"привет"},{"id":"30","text":"restart"}],a,c = temp.length;
for(var i=0;i<json.length;i++){
a = json[i];
for(var m=0; m < c ;m++) json[i].id == temp[m].id && (a = !1);
a && temp.push(a);
}
alert(temp[2].text)