The Last Jedi builds upon Rey and Kylo's relationship in a dramatically-satisfying way--teasing a change of sides for the both of them--while highlighting the similarities between their perspectives.
While the dynamic between the two of them matures, Finn is left with little to do but run around looking for ways to help, and Poe is disregarded as a hot-headed flyboy simply for wanting to have a plan. Why doesn't Admiral Holdo tell the rebels her plan? Why should they follow a leader who appears to disregard the concerns of her experienced personnel? Why doesn't Poe and Finn tell Holdo about their promising plan? Why does Luke leave it to the rebels to guess that he is making a distraction at the end? Lack of communication is a major player in the film, though unjustifiably so.

Speaking of Luke, I understand Mark Hamill's initial reservations to his character's approach, and agree with his fundamental disagreements regarding it. The Luke Skywalker that we all know and love would never isolate himself from his family and friends, especially if he felt guilt about personal failure. That guilt would only make him work harder to find a solution alongside his allies. The "grumpy old-man Luke" is an interesting concept, though I don't find the reasons in the film to be convincing enough. That being said, Hamill's performance is a major highlight of the movie, and arguably his greatest performance yet.

Thematically, The Last Jedi is all about coming to terms with defeat; a bold and admirable direction to take the series in, though considering its blatant disregard of Snoke and its teasing of a major battle and lightsaber duel at the end, it is fair to say that the filmmakers took the theme too far by making the audience have to suffer the acceptance of disappointment alongside the characters.