Hmm. So I... Hm. I have a lot of thoughts and feelings to discuss. This is probably going to turn in to quite the review. It's one of those ones where I really don't know how I'm going to fall on the scale when I grade it at the end. I'm just going to start writing, and I'll get to my thoughts through that.
Cons:
If I were to review this episode by comparing it to the milestone 200th episode, I might look on it with some disfavor. Why? Well, this was an episode centered around the nuclear Winchester family. So, the focus was on Sam, Dean, John, and Mary. And I'm cool with there being an episode focusing on that dynamic, and it's neat that Jeffrey Dean Morgan was able to come back and all that. But the 200th episode managed to focus on the themes of the show as a whole. It focused mostly on Sam and Dean, but it referenced everything from Adam to Destiel to Chuck to the Samulet and so much more. It was an episode that managed to be about the ever-changing, twisting road that is Supernatural over the seasons. This episode, by contrast, was kind of a "taking it back to basics" sort of episode, that encapsulated the original, long past, aims and themes of the show. Not a bad idea for an episode AT ALL. Just an interesting choice for such a big milestone. Barely any Cas, no Jack, no other recurring guests or even mention of them really... it felt like this episode existed in a time capsule, a little bubble that could have landed anywhere in any season and felt just as appropriate. Is that a bad thing? I don't know.
The elephant in the room here is that John Winchester was a demonstrably abusive father. Supernatural demonstrates that, but ever since his death back at the start of Season Two, the show has been inconsistent in how it's treated the memory of this character. It makes perfect sense, and is indeed good, that Sam and Dean themselves would have conflicting emotions. I don't mind that sometimes Dean brings up how awful John was, and other times seems to canonize him as some sort of a saint. That makes sense for his character. I don't mind that Sam has forgiven John for everything and has his own regrets about their tempestuous relationship. But here's where it gets a little more shaky for me. Think about it: from John Winchester's perspective in this episode, he's in 2003. He and Sam are not on speaking terms. He is continuing to emotionally and perhaps physically abuse and manipulate Dean, who idolizes him in an unhealthy way. Sam and Dean can remember their dad with rose colored glasses, but a more interesting, and more real, interpretation of the John Winchester that they should be meeting in this episode would be one that's so full of anger and grief that he lashes out at his two sons and can't accept Sam wanting to do anything outside of the hunting life.