The difficult part of tanking (and healing too) is getting the job done when you don't out gear the fight. Unlike DPS, who always have to maximize DPS (or at least, they feel they must for fast and easier clears) a tank or healer will find their job easier and easier every time they obtain upgrades and every time they find something that works a little better. Also, a low DPS can be carried. Same as healer. That's because both these roles are 'throughput' roles where you obtain a certain level of throughput, you are good to go. Not so with a tank. Your skill/quality can't really be quantified and therefore can't really be carried through. Especially if it's the mechanics themselves that can't be handled.