Hopes are high for Rogue One. Based on the trailers, everyone expects that it will match if not exceed the standard set by The Force Awakens. However, Rogue One is also the continuation of a process started in Episode VII: rewriting both the past and the future of Star Wars.
Almost everyone is at least aware of the Star Wars movies, even that small minority who have never seen any of them - like Star Trek, it's become embedded in the Western pop culture matrix. More dedicated fans (or attentive parents) are familiar with the various computer animated TV incarnations such as The Clone Wars or Rebels. However, the general population is completely unaware of the full epic sweep of the universe in which these stories exist, a universe documented in comic books, video games, cartoons, and hundreds of novels.
Or rather, in which they used to exist. The Force Awakens marked the end of the Star Wars Expanded Universe.
The Expanded Universe is exactly what it sounds like: an extended view of the milieu in which the Star Wars movies take place. The SWEU chronicles a broad span of time, dealing with everything from the beginnings of the Star Wars universe (literally the beginnings, starting with the formation of the galaxy at 13,000,000,000 BBY**) to hundreds of years after the events of A New Hope. Continuity and canonicity for the SWEU are documented through the aptly named Wookiepedia, Lucasfilms' wiki for all things Star Wars.
The Force Awakens indicated a clear point of divergence from the post-trilogy future established by the Expanded Universe. In the SWEU continuity, Han and Leia have three children, all of whom become Jedi Knights. One of them, Jacen Solo, helps to defeat the alien Yuuzhan Vong when they invade the New Republic in 25 ABY, although sadly both his brother Anakin Solo and his father's friend Chewbacca perish in the struggle against the invaders.
And that's just the tip of the SWEU iceberg. We haven't even started on Mara Jade's story, not to mention the hundreds of other supporting characters involved in the events of the Expanded Universe.
Those characters aren't gone - Disney isn't foolish enough to abandon the income stream being generated by the existing material - but they're no longer canon, which in a lot of ways is the same as a death sentence as far as hard core geek fans are concerned. The SWEU has been rebranded as Star Wars Legends, and Wookiepedia entries now distinguish between Canon and Legends as necessary.
However, as Disney follows through on its plan to release more films like Rogue One that are set in the Star Wars universe, along with the next two films in the new trilogy, the two will diverge more and more. I expect that the Legends material will be quietly eliminated by Disney in the fullness of time.
Unlike most children, I was raised to think of Disney as a sort of evil empire. My mother, who was quite a hard core fan herself, felt that Disney was a perverter of truth, whose adaptations of classic children's fantasies had very little to do with the original stories, and everything to do with making a quick buck at the expense of the source material.
However, the end of the Expanded Universe puts the House of Mouse into a whole different league of villainy. In this case, Disney has achieved a massacre beyond anything the Galactic Empire, the Sith, or the New Order could ever dream of accomplishing, wiping out thousands of planets, countless alien species, and generation after generation of sentient beings with a simple corporate memo.
Welcome to the Dark Side, Mickey.
* Something about the phrase "next prequel" sounds wrong to me, like military intelligence or government assistance.
** BBY - Before the Battle of Yavin. For non-fans, the Rebellion base that is being threatened at the end of A New Hope is on the fourth moon of Yavin, so the encounter in which the Death Star is destroyed is commonly known as the Battle of Yavin. Events following this landmark event are After the Battle of Yavin: ABY.
*** There's a scene in the Star Wars takeoff Spaceballs where Dark Helmet says, " I am your father's brother's nephew's cousin's former roommate." In the context of the Expanded Universe, that's no longer a joke.
For readers who may not remember Mr. Antilles, he's the only Red Squadron pilot other than Luke Skywalker who survives the Death Star attack. In a coincidence of names, Wedge becomes a member of Rogue Flight, Luke's squad from the Battle of Hoth. (Which is Canon.) Rogue Flight goes on to have an extended life in novels, games and comics as Rogue Squadron. (Which is Legends rather than Canon.) I can only guess as to any possible connection between Rogue Flight and Rogue One - I'll have a better idea once I've seen the movie.