Obviously in certain configurations it looks like a military rifle, which I could understand some people being spooked by, since military rifles are more closely associated with killing people than a typical hunting rifle. That doesn't necessarily mean that's why people buy it, though. My point was to respond to the rather common argument made by the GP that the only things that make the AR-15 different from other semi-auto rifles are cosmetic factors -- the fact that some people find it to look "dangerous", or scary. It is people on the gun rights side of the debate who are arguing the only reasons to buy the gun is the way it looks, which makes it sound like people buy it to look dangerous.
As you point out, functional differences are, in fact, a part of the gun's appeal. To what extent those differences make it easier to kill lots of people quickly seems like a reasonable subject of debate. It's disingenuous, though, when people dodge that debate by claiming that those differences simply don't exist and that the gun just looks different.