Enjoyed It, But S.T. Ripped Off Stephen King So Hard...
...he had to go find a police station to report a mugging.
A good half of the premise was lifted from King's novel, Firestarter, including a main villain (Dr. Martin Brenner) amalgamated from Firestarter's "Cap" Hollister, John Rainbird and Dr. Joseph Wanless, along with a sinister scientific government organization, Hawkins National Laboratory, standing in for The Shop, and even an originating psychedelic drug trial like "Lot 6" responsible for a protagonist's psychic powers. Other elements seem borrowed from King works like Christine (Jonathan and Nancy courageously seeking to rumble with the Big Bad like Dennis and Leigh) and The Body (train track scene). The Upside Down closely resembled King's Lovecraftian shadow realm from stories like The Mist, Crouch End and even The Breathing Method.
To give credit where credit is due, the Duffer brothers expressly name-checked King in episode 6 of season 1, but the large number of borrowed elements from King made Stranger Things seem somewhat more derivative than referential. As always, ymmv.