What did the movie get right about the novel?

The Harkonnen in the movie are melodramatized to seemingly enhance their grotesquery and evil nature.

They are rivals of the Atreides, they do engage in abuse of their populace, but the Baron is not gripped by some skin diseases, there are no heart plugs, no bizarre oil baths, and no milking of some bound up cat to produce the antidote to a poison.

They do enslave people, torture them, engage in gladiatorial combat to the death, and they do plot against others for their own advancement. The baron is a very capable strategist.

I read the book a long time ago, but yeah, the Harkonnens are pretty obviously evil and gross in the book. The Baron's very fat... pedophile too.

Not at all, the miniseries is much better in the portrayal of the Harkonnens. The Baron is a very clever, sane, evil old man in the vain of Roman Emperor Tiberius, not a villain from the 660s Batman TV series as Lynch made him.

I actually think the movie did a very good job capturing the feeling of the book and trying to portray a complex novel up until Paul and Jessica flee into the desert at which point the movie goes to sh!t.

The Harkonnens are exaggerated, but not that much. It's quite clear they're appalling people. One scene in the book, the uncle requires that Feyd personally strangle all the women in the pleasure wing as a punishment for trying to kill his uncle.