Board Thread:Theories/@comment-27301168-20170507074839/@comment-27125793-20180428124715

I think the first thing that should be mentioned is that by "special ability", it is implied a unique physical attribute or power, not something like willpower.

In my opinion, the Attack Titan's power is the fact it's physically well-rounded in its abilities. It has great physical strength, while normally seeming to have a lean physique (not counting Grisha), allowing it to maintain great speed and agility. It's also shown great endurance, as Eren transformed three times during the battle of Trost, and during the Battle of Liberio, showing no signs of exhaustion or overuse afterward in the latter case. Since it tends to be well-proportioned physically, not being weighed down by anything (like armor in Reiner's case), and yet having tremendous strength and endurance, it allows the wielder to use their fighting capabilities to the max and then some, though this only with the assumption the wielder has fighting skill to begin with.