I love how they find ways to keep it fresh by adding new angles and spins. We've seen countless ghost episodes, and yet it never gets boring because there's always an interesting twist. In this case, it was really cool we got to see things from the ghosts' point-of-view. And, one ghost was becoming more and more powerful by preying on other ghosts he was creating and holding captive. It really upped the ante, giving us a superghost scarier than the rest.
And on top of all that, we have the quandary of Bobby. It's great that he wants to keep helping, but as Dean said, how can this end well? The boys' emotions are in a real tug-of-war, they're so happy to see Bobby again, but so afraid of what will happen to him. That was really moving, I got goosebumps when they were hoping to see "Bobby" appear on the mirror.
Annie was so kickass, I thought it was really awesome we were getting a woman hunter. And I thought crap, she's going to die. And sure enough, she was dead two minutes later. At least she got to be a kickass ghost for a bit longer. (I really hope we get to keep Sheriff Mills!)
Yep, that's why I love Supernatural: no other show makes me laugh, cry and cower behind my hands like it does. And this episode had it all.