Two differences from the novel stand out. The most obvious is Chani's story. I think the changes are good.

To give the woman more to do in the film than be in love with the man is consistent with modern ideas on good storytelling. Through her, the Fremen become a more multifaceted people, of differing beliefs and levels of belief.

Perhaps most importantly, it gives the story a needed counterpoint to balance Paul's growing fanaticism. Can't have every single character, born or otherwise, egging him on toward increasingly questionable decisions.

The change I don't understand and that annoyed me was that Paul's sister wasn't born. This compresses the timeline and it's a pet peeve of mine when movies make too much happen over too short a time span.

Paul needs to learn their ways, be accepted among them, be accepted as a leader among them, wage an effective campaign to disrupt spice production globally, decide not to go south, change his mind... all while falling in love.

The story plays out better over years. And Paul and Chani's rift at the end becomes more meaningful if they have a relationship of years instead of months.