Kathleen Kennedy confirmed to Variey a few weeks ago that Rogue One: A Star Wars Story would not have the traditional opening crawl to start the film but didn't elaborate as to what exactly the film would have. In a translated interview from French website TF1 director Gareth Edwards explained, in better detail, how the film will open and why the crawl won't be present.
"There is no opening crawl. But there is a prologue at the beginning of the film that takes place 15 years before the action of the film and that is how the action is exposed. Our film comes from the text of Episode IV…if we give the text, it would be endless. We really had to differentiate Rogue One from the rest of the saga."
While I'm sure the lack of the crawl is going to make some Star Wars fans angry it's quite apparent what Edwards and the rest of the team are trying to do. Rogue One: A Star Wars Story is the first in a new type of Star Wars movie so it makes sense that they would try to differentiate themselves the best they can.
The "prologue" that Edwards mentions will likely be the flashbacks we've been in some of the trailers featuring a young Jyn Erso and perhaps some of the details of the prequel novel Catalyst.
On a personal note, while I wasn't present at Celebration this year, the video they played before the panel would have been a fantastic opener on its own.