The Impala

Eric Kripke has said over the years that he had talked to people about what cool car to use. Ultimately, they came up with the Impala, which earned the nickname “Baby” throughout the show and in the fandom.

The Impala became a character in its own right, with an entire episode dedicated to the car. See the trailer below. The episode is titled Baby and is season 11, episode 04 from the car’s perspective. At the end of season 15, Warner Bros. gifted Jensen Ackles the Impala.

Quirky Details

In the pilot, Sam and Dean ask a question at the same time, and that timing became a “thing,” a few quirky things carried throughout the years. Check out the pilot clip of that scene, followed by a clip a few years later where “Becky,” a fan comments, “Oh, I love it when they do that.” To coin a phrase, these quirky details became a “solid” for the show. 

Humour

Eric Kripke is sharp, no doubt about it. That humour translated very well into Supernatural and even behind the scenes. The clip below shows Jensen going off-script due to Jared missing his queue. The clip went viral through fandom after the studio decided to keep it in and add it to the end of the episode they had been filming. The off-script clip was also added to the hard copy release. 
Humour really did carry the show. It struggled for over a year when the showrunner/creator left and took the jokes with him. Eventually, that got ironed out when they found someone with those complexities who was a match for Supernatural.