What I really appreciate about Supernatural, is the consistency of the show.
Having watched seasons 1-12, probably three times already, the one thing I've noticed is that the writers and producers have not fucked with the "formula," even dating back to the early days.
They open with a small history lesson from previous episodes, then and opening scene, followed by the Supernatural logo, and then right into the intro of the story. And it has been the same way from the beginning. They didn't fuck with that, and I think people appreciate it.
The best part really is the brief "history lesson" at the beginning, because even if you haven't watched the show before, you can pretty much pick up what you need to know at the very beginning, and the episode will make sense.
The one thing that has changed is that the show was a bit scarier in seasons one and two. They did a lot of haunted places and ghosts, etc., with creepy and scary monsters and places.
Now the show has focused mainly on angels and demons, and more so on the established characters.
I'm not saying it's a bad thing, because you can tell that people absolutely love Castiel, Crowley, Rowena, etc. But it has now taken on the feeling of a daytime soap opera, more so than a show about fighting monsters.
In the early seasons, we learned about wendigos, and shapeshifters, and all these other funny monsters that I have never heard of before, which was fun to watch.
Not saying the show is bad now, but the focus is just different.
I do have to say, though, that even though it has been on for 13 seasons, the content is still pretty fresh - all things considered.