The jury is still out, but the emerging fan consensus seems to be that Denis Villeneuve's Dune is the best adaptation of the classic sci-fi novel thus far. The original Dune is a thoroughly complex book, beloved for its detailed worldbuilding and complex interstellar politics, but the granular nature of its fictional universe has historically left many skeptical as to whether a successful adaptation was even possible.
The latest Dune film happily proves pessimists wrong by capturing the essence of the setting without getting too bogged down in the details. Audiences typically don't come to the movies to be lectured about fictional concepts with names like Kwisatz Haderach and Bene Tleilaxu, after all. While the movie takes care not to get too into the weeds with these concepts, there are still some important lore details left out that might help audiences make sense of the universe.
The Kanly Ritual Of Warfare
The practice of kanly is a ritualized, formal style of warfare or feud carried out between Great Houses in the Dune universe. Houses conduct kanly with one another to settle their violent disputes, while hopefully also placing guardrails on the conflict to prevent it from escalating into full-scale galactic war. A kanly continues until the enemy is completely annihilated, or a peaceable agreement is achieved.
The most famous, and important, kanly depicted in Dune is that between houses Atreides and Harkonnen. It's skipped over in the movie, so only fans of Dune the book will know about this arrangement the Baron has with Leto. The sanctity of the kanly is breached, of course, when the Emperor intervenes on the behalf of house Harkonnen to retake Arrakis. Such an act would ordinarily outrage the other Great Houses, which is why the deployment of Sardaukar to Arrakis has to be kept under wraps.
Nuclear Weapons
Where are the nukes in Dune? Although many forms of technology became de facto illegal after the revolt against the Thinking Machines, nuclear weapons were not one of them. Many Great Houses continue to stockpile nuclear weapons, referred to as Atomics in Dune, but their usage is highly restricted.
For one, the offensive use of atomics against human targets is vehemently disapproved of by the Landsraad -- grounds for the permanent exile of a Great House from the Landsraad at best, and full-scale planetary annihilation at worst.
The Landsraad
The Landsraad is the central governing authority in the Dune universe -- an assembly of the known universe's great houses, overseen by the Padishah Emperor of House Corrino. The Landsraad is the principal forum for contact and negotiation between the Great Houses and is theoretically supposed to balance the power of the imperial house.
That all sounds orderly and civilized, but the Landsraad is, in reality, central to the deadly realpolitiks of Dune's universe. Essentially, the Landsraad exists to afford strength in numbers to the Great Houses. Should the imperial house betray the galaxy's uneasy peace, the combined strength of the Landsraad's houses would theoretically be able to challenge the Sardaukar -- thereby establishing a precarious balance of power.
Sardaukar Training
The film goes to great lengths to stress the formidable combat abilities of the imperial Sardaukar, showing their anointment by blood on Salusa Secundus, as well as their religious conditioning. This scene does an excellent job of conveying the essence of Sardaukar training, but omits some of the details, probably because they were too wild for Dune to show.
For one, the dead men whose blood is used in this scene's ritual are likely Sardaukar hopefuls who didn't make it through training. Sardaukar are conditioned for war from birth, with only a select few surviving to the end. Secondly, Salusa Secundus isn't just any planet -- it's an intentionally inhospitable dumping ground for House Corrino's prisoners, with the aim of creating a hostile world where only the strongest survive to become Sardaukar.
Other Bene Gesserit Abilities
The Voice, the ability of Bene Gesserit uses to compel people to perform certain actions through uncanny powers of suggestion, is covered extensively in Villeneuve's Dune. But while the voice and some awesome Dune outfits are depicted extensively, there are other important Bene Gesserit powers the movie doesn't show.
One such example is the formidable combat prowess of the Bene Gesserit order. Called the Weirding Way, Bene Gesserit adherents were trained rigorously in martial arts. Their combat abilities, combined with their near-supernatural ability to regulate the minutia of their bodily functions, meant that they could confidently challenge even imperial Sardaukar in hand-to-hand combat.
Dune's Projectile Weapons
Keen observers of the latest movie might notice that although projectile weapons are present in Dune's setting, they don't seem to use firearms as we know them today, but there are plenty of swords. What was it, then, that the assassin fired at Duke Leto, and why are those sorts of weapons preferable to modern-day firearms?
The answer lies with personal shields, as is the case with many tactical considerations in Dune. Since shields are capable of reflecting most high-velocity projectiles (like a bullet), weapons such as dart guns or Maula pistols which projected slower, often poisoned darts became favored over their archaic predecessors.
The Legacy Of The Butlerians
One of the most notable twists to Dune's setting is that "thinking machines" are harshly outlawed, with everything from the simplest pocket calculator to the most advanced artificial intelligence being forbidden by rigid cultural stigma. That explains the need for spice and complex biological conditioning, but how did it come to pass?
The answer can be found in the acts of the Butlerians, those who rose against the machine gods many thousands of years before Dune takes place. However, theirs was not a Skynet-esque military struggle but a religious and spirtiual one. The Butlerians were less concerned with machines enslaving mankind than they were with the erosion of the human soul, so the "gods" of machine logic were cast down in a wave of galaxy-wide hysteria.
Lasguns vs. Shields
The Dune film correctly depicts lasguns, those dangerous rays of blue light, as a dominant battlefield force. Capable of slicing through basically any man-made material, lasguns are also often compact enough to be carried by individual soldiers, making it by far the most commonly used and effective ranged weapon in Dune's universe.
Why don't ground forces just laser each other to death, then? Well, they certainly would were it not for the deadly effects of the interplay between lasers and shields. If a lasgun fires on a shielded target, the interaction between the two energies triggers an explosion of nuclear magnitude. Indiscriminately firing a lasgun into a crowd of potential shielded soldiers is a surefire way to detonate the entire battlefield, making the lasgun extraordinarily dangerous to use.
Mentats
If computers are illegal in Dune, then how do governments administrate on a planetary, even galactic scale without so much as a calculator at their disposal? The answer? Just train human calculators instead! This actually comes up more prominently in David Lynch's film version, which is one reason that Dune movie might not actually be so bad (although he didn't have to make their eyebrows like that).
These individuals, called Mentats, were subjected to intense psychological conditioning (and drug use) that granted them preternatural abilities of cognition and memory. Mentats are indeed depicted in the new Dune film, but they aren't ever referred to as such. They're those guys who roll their eyes back into their heads and spout out a bunch of numbers from time to time.
Additional Spice Effects
The latest Dune film does an excellent job of explaining why exactly spice is so critical the story's setting -- it's essential to ensuring safe faster-than-light travel. However, the specifics of how, exactly, it enables this, as well as some other effects of the melange are glossed over by the movie.
Firstly, the most important effect of spice is prescience. Navigators of the Spacing Guild dose spice so heavily that they mutate permanently into strange forms, and use the foresight afforded to them by the spice to plot safe courses. Additionally, the movie doesn't mention that spice prolongs life to unnatural lengths -- in some cases allowing users to live triple the lifespan they would normally experience.