Stealth has it's own level system. The stealth level is dependent on the rogue's level. If you are level 1, a level 80 will see you no matter where you are in stealth. If you're a level 80, a level 1 will not see you in stealth almost at all. There are items and abilities that will increase your stealth level. For every 5 points of stealth strength, you get one level. For example, the rogue ability [Shadow Walk] will give you +20 stealth, which means you get the stealth strength of a rogue 4 levels higher.