Some good answers here and some that believe stereotypes. I have been around dogs my entire life and have only had concerns about 3. Dalmatian, Chow and a St Bernard BUT I have rarely met a dog that I did not like. The Dalmatian for no reason bit the face of the niece of a friend of the family and had to be put down. This dog was aggressive and a constant problem. With this said ownership has a lot to do with behavior but as with humans there are those that just seem not to be wired right. This is where good breeding practices come into play and I greatly disapprove of backyard breeders.
Since Staffordshire terrier (Pit bull) seems to come up a lot, let’s look at a trait of theirs that is greatly exploited. Staffies aim to please its owner or handler, and if you can stomach watching a dog fight, you will see the handler in the ring encouraging the Staffordshire to fight. When they don't encourage the dogs will stop fighting and sometimes start wagging their tail and want to play. The Staffordshire has a respectable past and only became a problem when jerks started owning these dogs for demeaning purposes. Even the dogs seized from Michael Vick have had a second chance at life and some are now therapy dogs or live in homes. Only one was put down to aggressive behavior.
Any dog has the potential to turn on you and some of the misrepresented ones are the best family dogs to have around. We can all remember the uproar over German Sheppard’s, Rottweiler’s and Dobermans now with the attention being on Pits. And Whitney05 is right in that Staffies were trained to be aggressive against other dogs, not humans. That is why many that have been used for fighting fit in well after rehabilitation with a family.
The most decorated dog in military history was a Staffie, Sgt Stubby, was in the trenches for over a year and warned fellow soldiers of oncoming attacks and even helped capture a spy. He met with Presidents, other dignitaries and was made a lifelong member of American Legion, Red Cross and the YMCA.
We condemn dogs that have been greatly exploited by unscrupulous, vicious, mean people and many of these dogs would have been great family members if they had good owners.