It seems like a lot of y'all liked it, and it felt pretty reactionary to me so I'd love to hear an alternative perspective.
Beyond the idea that it's a film about imperial colonial extraction from which we only get the perspective of the empire, it really feels like the presentation of the lifestyle of the royals seems very sycophantic, very deferential.
Like the royals don't experience lavish personal consumption or luxury, no sex slaves, no hedonism, no fun at all really, they're all just earnest and stoic hard workers. The representation of the ruling class is that maybe your bedroom might be a little bigger, but they're just as put upon as the rest of us because of all this duty they're so concerned with. It seems like the take-home message is that any material benefit of being in the ruling class is trivial, but the accompanying responsibly is a terrible burden.
(I haven't read the books and don't plan to btw.)
So can someone explain why they liked it as a leftist?
The core of the leftist critique will become more and more obvious the longer the series goes on, especially if they decide to move past the first book. In the film we see it when Stilgar meets the Duke. The Duke is the incarnation of capitalism with a smile, the nice liberal. He's not going to hunt down the Fremen like dogs, but under no circumstances is he going to stop spice extraction. He still views Arrakis as his fiefdom, his by right of the Emperor and he feels free to do what he will. The Fremen just want to be left alone and for the imperial extraction of resources to stop. House Atreides is not willing to do this, and in the grand scheme of things makes them just as bad as House Harkonnen. In fact, there's a case to be made that the Atreides are actually worse because they appear nice, but this case will become stronger when you watch part 2.
Likewise, the violence of the Fremen is show to be materialist in nature. They are a hard people, sharpened like a knife by their environment and external circumstances. Their revolutionary violence against their oppressors is shown as just, and it is obvious that if they are to overthrow the yoke of imperial rule they must resort to incredible violence. Nobody debates this fact, nobody is squeamish. This is simply the rules of the game. To defeat colonial oppression violence must be used. That is a very leftist take.
There's also the larger theme of Paul seeing a horrible, violent future and trying everything in his power to stop it but deep down knowing he can't. This is a very historical materialist point—that even Great Men cannot stop what history has baked in, that "Men make their own history, but they do not make it as they please; they do not make it under self-selected circumstances, but under circumstances existing already, given and transmitted from the past. The tradition of all dead generations weighs like a nightmare on the brains of the living." Paul's Jihad (which the movie is too cowardly to name but shows you in that vision of the Fremen on Caladan) is unstoppable, the doom at the end of the tunnel, much as things like climate change or nuclear war are for us. Try as we might, we know the end of our world is coming, and all we can do is watch.
This is what I was looking for, thank you
You didn't get the part where Paul is the tired white savior archetype? Who civilizes the savages and uses them for his own ends?
No it's subtler than that. Paul is the critique of that stereotype, since he ends his journey broken, alone, blind in the desert having lessened a people and unleashed genocide upon the galaxy. It's pretty explicit that the white savior trope is bad and only ends in evil, destroying both the savior and the once great people he used towards his own sick ends.