The techniques of aikido are not unique. Every jujutsu, chin'na, te, tai chi chuan, iaido for that matter, will have most of the same techniques. The human body only locks up, or goes into catastrophic failure of equilibrium, in so many ways. The difference is how and why it is done. An essential definition that I focus on is
attaching to the opponent's energy. Attaching to energy may be invasive or evasive. In all cases it is by definition non-resistant, even if you initiate the movement of energy you are attaching to (stimulating a reaction). Even this principle is not unique, it just classifies aikido as an 'soft, internal' art. Another related non-unique, defining factor is the intention not being to "win" but to transcend victory and defeat, to remove fear and desire, and to de-escalate violence. Attaching this to aggressive energy tends to dissipate it by removing contention. The 'ethical' dimension can just be a tactic, but it can function on all levels as a fundamental life principle.
What has been will be again, what has been done will be done again; there is nothing new under the sun. Is there anything of which one can say, "Look! This is something new"? It was here already, long ago; it was here before our time. (Ecclesiastes 1:9-10)
Aikido is one particular incarnation of a perennial mode of interaction with violence, with the typical Japanese cultural style of simplicity and artistic balance, organization and efficiency, which can be described as "elegance". That character may best define what is unique about aikido.