The storyline with Nick is interesting because Lucifer corrupted him. Lucifer's whole shtick is that he is appealing and following him makes people feel good as they head toward Hell. That said, I don't really get the story. This isn't Nick's body. Nick shouldn't be in there any more than Jimmy is in Castiel. It's another example of the show contradicting established mythology this season, and why I think it is time for a new showrunner.
I don't think that it would work to reveal the supernatural stuff to the world. Supernatural works as a truckstop horror story, but whenever they try to go bigger, it falls apart. Kripke understood this about the show, but others don't seem to. The more you see, the less scary something is. The bigger you go, the less impact it has. The more you bring people back from the dead, the less anyone cares about life or death on the show. This is why the Heaven/Hell stuff became almost comedic and silly after a while. People in suits talking about how powerful they were while accomplishing nothing. It was so much more powerful when we got bits and pieces, but never saw what was really going on.
Season 7 was the closest we've come to seeing the world let in on the big secret, and it was a mess. We later had the President and big government agencies, and it looked ridiculous. Whereas one FBI agent tracking Sam and Dean in the early seasons felt like a real threat.
One of the most overlooked parts of storytelling these days is just establishing boundaries. Now that we can show anything on screen by using computer animation, everyone wants to show everything.
The 300th episode had some good moments, but I'm not sure it was really on the mark. It captured how far Sam and Dean have come, but it didn't capture where John was at that time. He didn't act like the John Winchester of that era. I don't think that he needed to be all angry and abusive, but I think that he definitely would have questioned some of the decisions that his boys have made.
It did feel rushed (maybe we didn't need the whole storyline with the kids and the town). Parts felt too wacky (Sam's Steve Jobs costume). There was some good emotion, and I think that Samantha Smith did a great job of making us feel a connection with someone whom she barely worked with 14 years ago. That said, the overall execution of this scenario didn't feel... real. It seemed more fanfic-y than, say, the time travel episodes where Sam and Dean interacted with their parents in earlier seasons.
I get that they probably didn't have JDM for very long and it was easier to play it out on one stage, but it felt off to me.