One other thing that I find to be technically unique in Aikido (I have never seen it's equivalent in any martial art other than certain kenjutsu styles (the very styles that influenced Aikido's design I suspect))…is the concept of omote and ura (or Irimi / Tenkan) being reinforced.
Omote and ura are THE tools to deal with multiple attackers. You use those variations to control where uke is going to land in order to protect yourself against multiple attackers.
Now while I am not saying no other art can handle multiple attackers, I AM saying that only Aikido (and some Kenjutsu schools) (IMO) make a concerted effort to move in a way as to place your body in a protected position at all times.
The tenkan movement itself is almost unique to Aikido (and some Kenjutsu schools). Almost in that I have seen a few martial arts styles that do something similar (no the same, just similar). But then again few other martial arts make a concerted effort to either be in uke's shikaku (dead angle) or extend uke over his/her shikaku (maybe Judo -- partially).