I think something that tends to get lost whenever "stopping power" topics comes up is the distinction between stopping the attacker and killing the attacker, as a few other posters have mentioned.

The purpose of a self defense gun is to stop a violent attack. The attacker may die as a result, but that is not the goal.

Adequate penetration is one factor. Yes, a round that penetrates deep enough (12-18" if you follow the FBI protocols) will be more likely to stop an attacker than a round that doesn't, but not all wounds are the same. A round that penetrates with a deep, but narrow, wound track, like FMJ, may end up killing the attacker, but because of the elastic nature of flesh it may not necessarily stop the attacker quickly enough. On the other hand, a round that doesn't penetrate deeply but expands or has a cookie-cutter effect, like a JHP, may actually cause more trauma and stop an attacker, even if it's not necessarily a lethal wound.

Of course, a round that both penetrates adequately and expands/cuts along the way will be even more effective at stopping an attacker, which is why typical service calibers, like .38 Special/.357 Magnum, 9mm, .40S&W, .357Sig, and .45ACP, are recommended over the .380ACP.

But even then there's no guarantee. All self defense handgun choices are a compromise in one way or another. While most people can probably carry a service-caliber handgun most of the time, that may not always be feasible. People have to make a choice as to what compromises they're willing to make. A compact, more easily concealed gun in a lesser caliber may be a better choice than a larger gun in a service caliber depending on the situation.

Just my opinion.