When players like Kitru and KeyboardNinja claim that Shadows are superior tanks, what exactly is meant by that? I've read the statement 'eke out superior performance' many times, but it tell me nothing. At the highest levels, why is a Shadow tank supposedly better than a Guardian or Vanguard? Complexity aside, what does it do different that DPS and healers will appreciate?
DPS will appreciate not having to hold back for a second.
Healers probably very little, many inexperienced healers hate Shadows because of how spiky they are. They may require the least healing of the 3 but they require the most attention. They do have the self cleanse and awesome CD in Resilience which can be a life saver when used appropriately. They are also the best tanks by far for I/E heavy bosses like Stormcaller, Jarg and Zorn.
Basically, all tanks can handle all content in this game. Some tanks handle certain things slightly better. Shadows with I/E attacks or Guardians with enraged bosses or serious "OH $#!T I only have 2% HP left" moments. Really though, the difference is pretty small overall so whatever you enjoy the most is going to be the best tank for you because you will spend the time to get it right.