Several times throughout the movie, Loki uses places the point of the glaive on a victim’s chest near their heart, and a blue fog passes from the stone to infect them: an electric blackness creeps upward along their skin from their chest until it reaches their eyes, which turn fully black for a moment before becoming the same ice blue of the glaive’s stone, and we see that the victim is now enthralled into Loki’s servitude.
You have heart.
The glaive is very, very terribly designed for this purpose.
It freaks the victim out (or should, anyway)
Look at that damned thing. It looks like an elven shiv. A can opener for human flesh. When a victim sees it coming, he will reasonably presume it’s going to split them like a fresh-caught fish, and do whatever he or she can to flail away from it. See how Loki has to grab Hawkeye by the wrist? That’s because short of some sort of hypnosis, Hawkeye would not just stand there like that with Orcrist slicing towards his sternum. We have to backworld some sort of pre-enthrallment mind effect to explain why he’s not jerking in the other direction. As all great propaganda and persuasion masters know, you can’t approach as a threat, or the victim’s fight-or-flight might kick in and slam that window shut for winning their hearts and minds.
It might, in fact, slice the target open
Even if there’s some mystical roofie thing going on to calm the victim, if Loki had too much force behind his approach, or someone bumped either of them, the glaive could go into the victim, causing a shock of pain that might wake them up before the enthrallment could take place. Or worse, it could actually damage the heart and kill the victim, which is counter to Loki’s goal.
It requires precision, control, and time
To avoid the disheartening of an intended victim, then, Loki has to grab them, momentarily hypnotize them into calmness, and carefully ease the thing up to the target, and hold it and them in place for a few. Imagine a button on a keyboard that had to be touched with feather pressure, or it would brick the machine. This would not be a great keyboard. All these are expensive dependencies, and the time it takes is time for onlookers to intervene (or to somehow incapacitate the victim to save them.)
It tips its hand
OK, fine, the glowing-blue eyes might be an unavoidable side effect of the “tech”—and yes, I understand it’s very valuable narrative purpose to signal enthrallment—but if you were designing an enthrallment tech, you’d want to avoid such an obvious “tell,” especially right there in the main location people target when looking at other people. 
A redesign
So there are a lot of ways this is less than ideal. Fortunately we don’t have to call iGlaive and tell them to shutter operations. I think we can fix this in one of a few ways.
Soften the industrial design? No.
The glaive needs to stay looking evil, and being sharp and pointy helps with that.
1. Have the glaive pull them in
A cinematic hack might be to visually imply that the glaive helps with these problems. Imagine Loki approaching Hawkeye with the glaive outstretched, and the blue fog appears and pulls Hawkeye towards its point. The point of contact can glow slightly, implying some protection, and the crystal can glow to do its enthralling. Now it’s a feature, not a bug.
2. Go broadside
If for some plot or cinematic reason that wouldn’t work, you could have Loki use the broad side of the glaive against the chest of the person. Slapping it like an oar onto someone would be a fast gesture that wouldn’t need a lot of precision to get the crystal near the heart. It could even enable sneakier attacks from the side. It might prove cinematically problematic when enthralling a female character, but since that doesn’t happen on screen in The Avengers, it’s moot.
3. A new gesture
If Loki isn’t the broadside sort, you could keep the staff the same and redesign the gesture. The mind is the thing enthralled, so it’s tempting to have it located on a forehead or neck, but we can’t have Loki gesturing to the victim’s head, because then we lose the awesome moment near the climax when Loki tries and fails to enthrall Stark on his chest reactor. So let’s keep it cardiac. Maybe we can change the relationship of the glaive to the victim.
Imagine if he lays the glaive across his left forearm, (or better: cuts into his own skin, which would explain why he just doesn’t keep enthralling everyone in sight) which begins to glow with the blue fog, and he uses a pointing index finger to tap the victim’s heart. A finger-to-sternum interaction would telegraph a lot less danger, risk fewer victims’ lives, and enable speed with less apparent precision required. As above, it might be problematic to enthrall a woman without the audience going OMG BOOBS, but again, we’re saved from that problem by the script.
In many ways this is my favorite of the redesigns. It’s a Natural User Interface. With blue fog.
Any of those tweaks might help us believe in the interaction and useful for us to keep in mind: requiring great precision of our users only slows them down and keeps them focused on the interface rather than their goals.