Guardian Dogs or Fighting dog types. Its hard enough raising the first dog without worrying about aggression to people or dogs. Of course any dog can be aggressive, but I think its best to take the safer route and avoid the breeds bred for it.
Independent dogs, Saluki, Husky, Mals, and even Greyhounds, especially if someone is looking for their Lassie or classic family dog. They don't always do their obedience thing well and are not the classic dog that people think about.
If they really understand the dog though, I don't think either catagory I posted is really out. If someone met a few Rotties and knows its the breed for them, fine, but there are additional concerns (size, bybs, aggression) that could be more easily avoided with other breeds. Similarly a hound would be ok if the person getting the dog truly understands that they are not labs or collies or something like that!