What are some unacceptable behaviors between your rover and your own personal dogs?

Even with a meet & greet, those behaviors are not always apparent. I don't recall any guests acting exactly in the way you described. However, there have been what I'd refer to as mismatches. If a dog acts aggressively, guarding resources (such as using growling, snarling, with stiff body language, showing teeth/raised hackles, & air snapping in attempt to control access to bowls, me, etc.) I'd nicely suggest to pet parents upon their return, that their dog may be happiest at home with a pet sitter who can provide one on one care. Play is different, but aggression is not welcome. I'm very selective, so dogs we care for are my dog's good friends who enjoy each other's company. All behave nicely towards each other.

NEVER leave the dogs unattended while they're together. You can't leave them alone at night. It takes a second.... Obviously, you can not expect that a dog will like everybody. Especially if the Rover dog doesn't know you or your dog. They are scared, anxious and stressed out. That makes everything worse. A lot of dogs will attack if they feel threatened. It's called fear aggression. You have to make sure you are able to handle them before you accept them. But they will always act different at meet and greet when the owner is around, than when they are with you all alone. It really can be very traumatic for the dog. It's normal for dogs to bark. But if you felt the barking dog's body language showed intimidation, then you have to keep them separated.