I don't see why folk want a dog that can be mean anyway. In many places if a burglar enters your home and your dog attacks, the dog will end up put down. Same scenario and you shoot the intruder, there is no issue.
I prefer a "watch dog" over a "guard dog"... My dog has no fence, has no chain, and doesn't even wear a collar. He is as friendly as the day is long, but alerts me to anything going on that shouldn't be. He never lets me out of his sight and goes everywhere I do. It isn't that he is well trained, it is simply in his nature.
Maybe I'm reading too literal. I believe any dog can be mean. Some breeds seem to have a tendency to viciousness but most aren't. Sysop, you are obviously a good owner. Your dog has you as its leader and has learned very well. It has learned that the majority of people are good and that friendliness is a good thing. I appreciate and applaud you for that.
I can think of two types of owners that are bad. The first is ones who train their dogs to be mean. The second is less obvious and is the type I am dealing with. He has a dog capable of being vicious and provides it no leadership. He just feeds it. Left to its own, the dog has grown into a pack leader and hunter with vicious tendencies.
I had an interesting discussion with a dog rescue group on Saturday. They class aggression into four levels, totally non-aggressive, aggressive towards other animals, somewhat aggressive to humans and having demonstrated vicious, uncontrollable aggression to humans. In one rescuer's view, once a dog got into the last level, it is almost beyond salvation and can never be trusted. He has one in that category. It will never be adopted out. He carefully controls its access to other animals and people. When he is around it, he always keeps an eye on it. He believes the dog could turn on him or his wife in a second. He just doesn't have the heart to put it down so basically he acts as its jailer. He treats it well but it is always contained. Using their classification, the boxer/pitt I am dealing with is playing with that last level. Since the owner isn't going to do anything to stop it, I am left with no alternative but to put it down if it doesn't flee at the sight of me on my property.
And yes, my wife and I are considering adopting a rescue dog. We have certain requirements first of which is no viciousness towards humans. We would be very concerned about viciousness towards other animals but have seen my son train his rescued rott/beagle to be submissive to their cat. (I thought that would never happen but can now see that his rescue has consistently shown a desire to please its people.) Like Sysop, our dog will almost always be in our company. It will be a cherished family member, not some creature locked outside, thrown food and left to roam.