I can't vote cause there's multiple elements that play a factor for me.
Definitely the gag, no tie-up is complete without one!
Role-play would play one along with what both captor and captive are wearing as it plays into my uniform fetish.
Definitely the sexual gratification or delayed stimulation ; cause it adds a layer of torture.

I like being helpless but I also like being in control so that plays a factor.
I also like the method of restraint used, anything involving handcuffs, leg irons, or chains in general!
And last but certainly not least the equipment as especially if it's part of a school uniform or athletic gear!
Everyone speaks in multiple languages...But gag talk is universal and a sock in your mouth is the perfect translator!