I think he completely deconstructed everything the franchise had been built up to be during the 1990s and 2000s. Not only that, but I think he confronted the very nature of franchises as a whole. The prequels (and the era they were made in) did a lot of damage by introducing elements of mythology, expanded universe, and lore that were never even considered during the creation of the original trilogy and weren't necessary at all. All the midi-chlorians nonsense, the word "Sith", the expanded universe novels, the video games, meeting Boba Fett as a child for no reason. It goes on and on and on.
In the end it all added up to the obsessive theorising that took place before and after The Force Awakens and revealed that, quite often, fans who love a long-running story often completely misunderstand it and begin to consider their headcanon as the way the story should go. Rey was definitely Luke's daughter or someone special, she was definitely going to convince him to come back and take on the First Order with a laser sword, Snoake was an important commander like the Emperor, the Jedi were still an exclusive and special group of magicians, the Force can only be used by a select group of people, etc.
I think Rian Johnson came in as a complete outsider, looked at the franchise as logically as he could, asked himself and J.J. a lot of very important questions about where the Star Wars story could possibly go from the place it was left in The Force Awakens, and wrote a script based around that. As we see at the end of The Last Jedi, the Resistance are basically 12 people on a little craft escaping an entire army. If only Skywalkers can use the Force then they're absolutely screwed, but Rey being "no one", for example, gives them hope: if "no one" can be a Jedi, then
everyone can be (as evidenced again by broom boy).
And if everyone can be a Jedi, they can form an army big enough to fight the First Order back.
One thing I realised watching through the films again last year after The Last Jedi is that, for absolutely no reason, everything seemingly has to be connected to something in the Star Wars universe. It can sometimes kill any enjoyment I have and makes the franchise feel uncomfortably exclusive. Faceless extras with no lines have lengthy Wookiepedia pages, one character seen briefly in the background of one film is apparently related to the destiny of a key character who appears in a later film, a lightsaber is a particular colour because of some deeply held trauma of the person who wields it - you get the picture.
It shows the power of the 1977 original that so much can come from one film in the end, but it was just a typical fairytale story that happened to be told in space. When fans give things an elevated level of importance and seriousness I think they can lose sight of what these films are supposed to do, which is distract you and entertain you for 2 hours. That's what I believe the sequel trilogy has really brought back to the franchise that the prequels absolutely didn't because they took themselves far too seriously, and the fact that The Last Jedi manages to inject so much fun to the saga while deconstructing and analysing how much "fandom" can actually
kill the fun of experiencing it is a huge achievement on Rian Johnson's part.
Just my twopence, though.