There are a couple lines that are kind of thrown in yet are actually quite important that clarify things.
In short...
Emperor Shaddam is a jerk and he has a lot of jerk families on his side - like the Harkonnens.
A lot of other families are sick of it and are starting to side with the Atreides. The Emperor knows this and launches a plot, in league with the Harkonnens, to destroy House Atreides - which is why they're sent to Arrakis.
The Bene Gesserit are like the Roman Catholic Church of medieval times and have their own agenda.
The Fremen are biding their time to strike against the power structure.
Spice is what is used to enable interstellar travel. So it's the most important thing there is.
Yep and I thought they did a perfectly fine job explaining all of this. The political motivations of the emperor sending house Atreides there and setting them up to be destroyed because he was worried about the other houses aligning with them was spelled out multiple times. The Duke (Oscar Isaac) spelled it out explicitly in a conversation with Paul. And then the Baron and Batista repeated the conversation a second time, explicitly. Then it was hinted at multiple times beyond that as well.
Same thing with spice. They literally said, explicitly and verbatim, that spice is the key to interstellar travel and hence is the most valuable resource in the universe. Twice, if I recall.
It's the portrayal of the whole thing as a slick political maneuver that didn't make sense to me.
Everyone with half a brain should have known what happened, and everyone involved seems aware of it as it was happening. So the idea that the supposedly powerful political faction allied with the Atriedes would take all this laying down didn't really track. In the context of a dumb sci fi movie it's totally fine and I enjoyed it, but the books take themselves far more seriously and I wasn't very impressed with how the whole thing was laid out. YMMV.
Except that any resistance there is is NOT powerful. It's in it's infancy.
And the Atreides are between a rock and a hard place. Refusing the assignment would be an
overt sign of rebellion. They HAVE to go.
As far as showing support for the Atreides, none of those families want the Sardaukar showing up on THEIR planet.
The whole idea is that the Emperor has sniffed out dissent in it's early stage and is making an example of it with the beat down he gives the Atreides.