Temperament, temperament, temperament! I don't want to see threat displays or anything more than a very mild shyness. (I'm assuming we're talking about my breed here.) Normal puppy issues can be overcome. But I want a dog that is fundamentally sound, preferably very friendly and interested in meeting people. I don't mind reserved and laid back -- not leary, just mellow -- so long as the dog is still responsive. Bright enthusiasm and toy drive is a big plus, but that's partly because I can't look at any situation from a totally non-training standpoint, and I like an enthusiastic worker. Mainly, I just want a dog that can walk into every situation with a good level of confidence, because its important to me that my pet be adaptable.
And I don't really want a dog that's off-the-wall dog-aggro. I will correct budding snarkiness as we go along, and I don't expect an unreasonable amount of passivity when approached by a rude or aggressive dog. But at the very least, I expect my dogs to learn that its not okay for their brains to go leaking out their ears when they see another dog. They're there on my terms, to obey my rules, not to beat up other dogs.