Rey was able to master Jedi Mind Tricks in The Force Awakens.
One of the biggest points of contention with The Force Awakens is that Rey was able to harness her powers of the Force with zero training. It was like she woke up one day and said to herself that she was going to control the mind of a Stormtrooper. How did she know how to do that, or that she even could do that? How was Rey or even Finn, for that matter, able to battle Kylo Ren, who had been trained in the ways of the Force? It just seems strange. The Last Jedi attempts to show at least a little bit of training.
The Last Jedi has character development.
In The Force Awakens, we never really get emotional for any of the new characters because there is pretty much zero character development. However, The Last Jedi takes the time to get into the heads of all of the major characters in a way that has never been seen in a Star Wars movie before, giving us some real emotional attachment to the characters, which makes us more invested in the story. The characters come off as human and unpredictable, which is refreshing.
The Force Awakens has inept villains.
Kylo Ren is only menacing during the very opening scenes of The Force Awakens and is kind of a joke for the rest of the movie. As for Snoke, he's rarely seen and when he is, it's through a hologram. The Last Jedi is able to pick up those pieces and show the conflict within Kylo Ren and the motivation as well as the quest for power. Snoke is even more scary in person and his use of the Force really makes him something to fear. J.J. Abrams never really set up the bad guys in The Force Awakens, there are moments, like Ben Solo killing his father, but those moments are few and far between.
There's not much Luke Skywalker in The Force Awakens.
Fans of Star Wars had been waiting over 30 years to see Mark Hamill back as the iconic Luke Skywalker and to see him reunite with the original gang, but that was not to be. We see Luke's robotic hand for a brief second and then at the very end of the movie we see Rey on Ahch-To ready to deliver Luke's Lightsaber to him. It was a surprise, and probably the only real surprise about The Force Awakens, but c'mon. The Last Jedi puts much of the focus on Luke Skywalker this time around while addressing why he was absent from the firs movie.
Rian Johnson wrote The Last Jedi by himself.
J.J. Abrams wrote The Force Awakens with a team whereas The Last Jedi benefits from the single vision of Rian Johnson. Abrams brings moments together while Johnson is a true storyteller and has made a dense and intriguing story that steps outside of the Star Wars box, which is something that The Force Awakens wasn't able to do. While some hardcore fans are angry that Rian Johnson took so many risks and made something that stands on its own, time will prove, much like The Empire Strikes Back, that The Last Jedi is an epic story and far superior movie to The Force Awakens.