Star Wars: The Aronson Canon
Only "A New Hope" and "The Empire Strikes Back", both pre-1997, are considered fully canon. And note, their novelizations are not considered canon, but only the films themselves.
"Return of the Jedi" (pre-1997) is canon except for two things:
First, Leia is not Luke's sister. She is not his sister, as this creates some continuity issues. By extension, there's never any talk of her remembering her mother, nor do Luke and Obi-wan speak of Leia as Luke's sister. However, she remains a potential second hope, as implied by Yoda in "The Empire Strikes Back". And she still ends up with Han. Luke understands.
Second, Vader can survive without his mask, but just doesn't want to. It's intimidating. Also, he opted to not get his flesh healed as a way of remembering what caused it. (And, deep down, he felt he deserved to have to wear that skin and mask.)
Midichlorians do indeed exist, but they're not the Force. They are, as originally intended, just a conduit or mediator between the Force and those who are sensitive to it. It is in fact the midichlorians that create and quantify this sensitivity. (This should have been much better explained in the prequel trilogy.)
The only way to know if someone is sensitive to the Force is to sense their use of it, or to do a midichlorian count. There are no other signs.
As Luke was growing up, there were subtle but unconscious uses of the Force in his life. He didn't really notice them, but his uncle and aunt did. They didn't talk about them.
The children of Force users have a higher chance of sensitivity to the Force, but not by much... apart from certain uncommon bloodlines, that is.
A person sufficiently trained in the Force can always sense when someone else trained in the Force is near. If they know this person, they can even recognize them by these means. They can also sense if this person is operating in the dark side. However, a master-level dark side technique can "cloud" or "blur" the Force making it difficult to clearly sense another Force user or their identity. There is, at best, only a vague sense of dread. Palpatine employed this technique to great effect.
There is no "Chosen One" of the Force. Anakin Skywalker had a father, but the man died when Anakin was little.
The dark side isn't a natural part of the Force but is instead a perversion of it. The term "side" is used because there seems to be a neverending war between light and darkness, and therefore the word "side" refers to allegiance in that endless struggle, not to a second half of the Force.
The dark side in itself doesn't corrupt the body in any fashion.
The Force isn't so powerful that a single user can pull starships out of the sky, even small ones. Yoda had to work a bit at just to lift an x-wing that wasn't otherwise moving. We see the Force's general limits in the first six official films. It's not as high-powered as it was portrayed in some Expanded Universe titles (along with the Disney films).
While a biological body can be cloned, its memories cannot be copied, and neither can its midichlorians. In addition, as of the end of the Galactic Civil War, no clone has ever been known to have a midichlorian count high enough to be Force sensitive.
The Jedi Order exists to unify the essential teachings of the Jedi, and to officially recognize a Jedi as such, but that's it. It doesn't control the Jedi beyond this. The Jedi Council is not officially a governing body, but simply a gathering of sages. They exist to help maintain unity among the Jedi, and to give the Order a face to the rest of the galaxy. However, their advice is often taken due to the deep respect held for them by most Jedi.
The Jedi are like samurai, able to marry and have children, but also tasked with keeping the peace.
There are various Jedi clans filled with relatives of one another. Each has traditions of its own.
There are different Jedi schools of thought, even if they all share a common foundation.
Jedi and Sith made use of long-range weaponry as well. Their lightsabers were kept mostly for short-range fighting and duels with others who had them.
The Jedi have no dress code beyond the requirement to be modest (sexually and otherwise). Kenobi, for example, wasn't wearing Jedi robes in "A New Hope". He was just wearing common clothing. Why would a Jedi in hiding walk around in Jedi clothing? Members of the Order are recognized only by their ways, powers, and lightsabers.
Potential Jedi usually aren't removed from their parents before at least their teenage years. But if even one of their parents is trained in the Force, removal is virtually never seen as necessary. Jedi keep ties to their families, as their training is much more like attending a boarding school than gaining a new identity.
The Jedi wore some degree of armor into battles. They weren't idiots.
Jedi were super rare. The prequel trilogy makes it seem like there were piles of them everywhere, but there were actually so few of them that it wasn't too difficult for people even before the Empire to dismiss talk of their powers as exaggerated, or in some places even to doubt their existence altogether.
The Jedi were not foolish enough to put sexually compatible people together as master and apprentice.
Some Jedi traditions would allow only one apprentice per Jedi, but others (most, in fact), allowed for more than one.
The Grand Republic is said to have been around for about twenty thousand years before it became the Galactic Empire, but it's hard to say. Its history stretches back into the thick fog of history.
The Republic has gone through many changes, some of them so radical that little more than the name and some degree of adherence to democracy was carried on through.
Kenobi's primary mentor was always Yoda. Qui-Gon Jinn never existed.
Vader and Palpatine both knew about Luke. (This is obvious from the pre-1997 version of "The Empire Strikes Back".) This is why Luke wasn't actually hidden. After all, he was placed with his family and kept his father's name. He simply wasn't considered a threat. After all, what were the odds that he'd even be strong in the Force? It took three years after the Battle of Yavin for Palpatine to finally decide that Luke was becoming too honed in the Force, even nearing the mastery of a Jedi, to be allowed to survive.
Even by the end of "Return of the Jedi" Luke didn't really have a great mastery of the Force. He was good enough to be a Jedi, but still young and without the wisdom that can only come through many years of training. He did progress rather fast thanks to his midichlorian count, natural aptitude, and help from the Force itself, but it wasn't enough to make him a true match for Vader. Luke won that duel only because of raw talent and strength, along with tapping into the dark side for a power boost. Vader wasn't prepared for the latter.
By the time of Yoda's death, Luke was truly the only Jedi left. There weren't Jedi in hiding that Yoda didn't know about, and even those who had renounced the Jedi Order had been killed by the Empire.
It's not common knowledge that Anakin Skywalker became Darth Vader. In fact, it was kept a secret for Anakin and Luke's sake.
The Sith Order is an ancient cult of dark siders opposed first and foremost to the Jedi. They were thought destroyed millennia before the time of the Clone Wars, and few people even knew they had returned until the end of the Galactic Civil War, when the truth about Palpatine and Vader was revealed to the public.
There was never a "Rule of Two" among the Sith. Palpatine had adopted ancient Sith teachings he found, reviving the Sith. Desiring no competition or discovery, he only brought on Vader to be a fellow Sith.
"Darth" isn't a title, it's a first name. The Sith didn't use "Darth" as a title.
There was never a Count Dooku or General Grievous.
The Clone Wars were a series of separate wars waged by clone armies against the Republic. The leaders of these clone armies were clones themselves. These leaders were ancient, non-Force-using enemies of the Republic. Over a period of decades each gathered its power before finally striking from different parts of the galaxy. A shadowy figure was known to have created these clone leaders and increasingly provided them with power, but the identity of this figure was ominously and frighteningly never discovered. It was Palpatine, who then used the wars and the fear they caused to transform the Republic into the Empire.
After taking power, Palpatine framed others for the Clone Wars.
Science and Technology
Lightsabers don't make use of crystals.
Kyber crystals never existed.
Holocrons never existed.
The Jedi and Sith aren't the only people who use lightsabers, but few aside from them do. Without the Force they're not very useful in a galaxy of long-range weaponry.
The reason so many planets contain a single biome and an environment compatible with most species is due to widespread terraforming over many thousands of years.
A ship at lightspeed can't hit anything while doing so because it's in hyperspace. However, while in this alternate dimension, a ship is still affected by gravity wells from normal space. Those of stars, black holes, and other highly gravitic phenomena cause a ship to drop out of lightspeed. Because of this, and the risk of dropping out of lightspeed into something solid or otherwise dangerous, a jump must be carefully calculated.
When a ship engages its hyperdrive engine, the ship only projects the illusion of darting forward in normal space. But what's really happening is that it's already entered hyperspace and is leaving a fading echo behind it.
Larger starships and many populated areas employ artificial gravity wells attuned to hyperspace. They're designed to keep ships from emerging from hyperspace too close. Some military vessels are even specially designed to project these wells out a great distance to use like a net.
Given the last three points, it's not usually possible to use lightspeed as a means of creating a projectile weapon.
Only the rare person has noticed something peculiar that spans the length of known history: there hasn't been much scientific or technological advancement since the earliest times. But even when someone notices it, they tend to forget they ever thought it. Only those trained in certain disciplines of the Force know the truth: it is the will of the Force that science and technology only grow at a slow crawl. But no one knows why.
Likewise, the Force has strangely willed that history isn't preserved very well over centuries and millennia. And again, virtually no one thinks anything of this. Whatever happened more than three or four millennia ago is often enough lost to legend and then eventually myth (if even that).
And again, likewise, it's only due to the will of the Force that cybernetic augmentation hasn't spread like an epidemic across the galaxy, and is common only in the replacement of crippled or lost organic parts.
Different alien species are virtually never able to reproduce with one another. But if they can, it's only because they share a relatively recent common ancestor. And if they have this commonality, the two species will probably also look very much alike. Humans and twi'lek, for example, are genetically compatible, and thus share a common ancestor. This also explains why they look so similar (aside from certain areas of the body).