It's simply very well written.
Stranger Things is about a small town in Indiana where young Will goes missing. From the beginning it's clear that something not quite normal is involved in his diappearance. The search for Will by his mother, his brother, a police officer and his best friends is the main plotline of the story. By focusing mostly on this storyline the show manages to tell a straight forward story in the middle of all the not straight forward stuff in the background.
Whatever happens, all side plots sooner or later play a part in the main story, the missing case of Will, f.e. the disappearance of Nancy's friend Barb makes her investigate and team up with Will's brother Jonathan. The appearance of the strange, homeless girl with a shaved head, who everyone just calls Eleven, based on a number tatooed on her arm, helps Will's three best friends to follow new leads in their search for Will.
Say what you want about the science fiction theme of the show, but it is (especially for this genre) extremely well written, intruiging and easy to follow.
It has great child actors.
Usually I hate protagonists who are children, especially in a show which is clearly written for adults. But in Stranger Things I loved Will's three best friends Mike, Dustin and Lucas who are all really cool. The way they investigate Will's disappearance, reminded me of how much me and my friends loved pretending we were private investigators as children. It brought me back into the mindset of those happy days. Also they are neither evil nor idiots like the children in many movies or tv shows. They are actually pretty smart.
Of course, for the likability of the child characters we have to thanks the child actors who give a fantastic performance. To find good child actors is really hard, which is why they often have to be replaced several times. But Stranger Things managed to find child actors who don't appear ridicolous in an adult tv show but do their job really well. And I don't only mean the three boys, but also the actor of Eleven.
It's mainstream sci fi.
Sci fi has always been popular with a certain type of people while the rest can't really get into it. Not so with Stranger Things. Stranger Things is sci fi for people who don't like sci fi. Which honestly is the best kind of sci fi. Which leads me to my next point ....
It's not too weird.
The biggest problem of many sci fi or fantasy shows, movies and books, is that it quickly becomes too weird and confusing to the point where the viewers and readers start wondering what the hell they are watching/reading. In the beginning Stanger Things of course is quite confusing but the show reveals and explains everything bit after bit and (in it's own supernatural logic) it all makes sense.
Sometimes shows try to invent something new which makes it difficult for the viewers to grasp. The trick is to take something old and give it a new twist, which is exactly what Stranger Things does. Parallel dimensions, telekinesis - we all know those concepts. Now we can watch them in a new setting and story.
Let's be honest. Aren't you getting a bit tired of the thousands of book adaptations, remakes, etc. which flood tv nowadays! I definitely do. It's so refreshing to watch an original show for once.
It has many positive characters.
Except the obvious evil characters like the monsters, all characters of Stranger Things are positive and can be role models in some way. From the kids, over the teenagers to the adults, none of them are evil. This might sound normal, but I am so used to shows like Game of Thrones where every character has a deep dark secret or an evil agenda, that it's uplifting to follow the paths of everyday heroes in Stranger Things.
It has Winona Ryder in it.
Because everything gets better if it has Winona Ryder in it.
Bonus: It's set in the 80s.
Just to make it clear: It's not relevant that the show is set in the 80s. The story could have been set into today's world or any other time and it would work just as well. The 80s are just a little extra for all the nostalgics and retro lovers out there.
So, if you haven't yet, go and watch Stranger Things :)