It's your gear + weapon, weapon dmg for every class in this game is HUGE most people you're grouping with probably have either their Zeta (ilvl135) or other high ilvl weapons. Once you catch up and get some more gear you'll be laughing at how easy you'll tank. Just be patient and it'll all come as a guy posted before, most people were running around in gear called "Slaying" in their right gear slots which is: Str, crit, determination which helps excel warriors (and other tanks) at tanking.