I had one major problem - Jack, the most powerful being, can just read Nick's mind. Even Cas can do that. So I had trouble getting past that since they should immediately have been able to know where Donatello was and what Nick's plan was.
Other than that though, man did I love that fight scene with Dean. Badass Dean is my favorite. And Jensen killed the scene where he is trying to keep Sam conscious. The way he spun away to hide his face/emotions when Sam was okay - wow.
I had trouble understanding what Jack did that was so bad to Mary. He is a homicidal maniac who was trying to bring back Lucifer. I have no problem shooting his ass, beating his ass, frying his ass.