I have to add that I personally feel Star Trek: The Motion Picture is one of the most Trek-like movies of the series.
Of all the Trek movies, it doesn’t feature an over-the-top villain-of-the-week (with no insult intended to some of the better villains), focuses primarily on the very human characters and their interactions between each other, and focuses on a very alien and seemingly unknown construct of mind-boggling proportions that defies human understanding.
Despite the re-use of the “lost human space probe comes back with a vengeance” theme as seen in a TOS episode, the entirety of the V’ger vessel was mind-blowing to me, totally alien, and imagination-prodding. And seeing the crew react to it, trying to wrap their minds around it and make contact with it seemed more Star Trek to me than Kirk getting into another brawl with another humanoid enemy.
Star Trek is about the unknown, about humans diving in head-first and seeing what’s out there, about how they handle themselves and each other, how they can grow as a result of new understandings. By the end of TMP, despite Decker being the one to save Earth, it was Kirk, Spock, McCoy, and doubtlessly some of the others who came out of it with growth of character. And like others have said, this was an important movie for Nimoy’s character too, his character’s growth especially, making it ironic that he thinks little of it.
Despite all the troubles the movie had getting off the ground, politics between the writers, Roddenberry, and so on, TMP speaks to me…and I’d welcome a new “cerebral” movie like TMP over the next attempt to recreate Khan.