Warning: Spoilers for Justice League
[Update: DrAwkward at Man of Steel Answers published an excellent examination of the claims read by Kevin Smith, concluding that, while many details may corroborate with known changes to the movie, the source publishing the differences is likely falsifying information. The credibility of the audience member claiming to have seen a test screening is also in question, but with a non-disclosure agreement in play, the fact that the screening would have shown an incomplete film, and there were likely multiple versions screened, it's difficult to verify if she's also lying, misremembers details, or merely saw a different, but similar version. Regardless, the explicit plot details read by Kevin Smith should taken with a massive grain of salt until more accurate details of Zack Snyder's version emerge. The original post is as follows]
In the wake of Justice League's disappointing critical and financial opening, the question of what the original version of the movie was supposed to be has been a big topic of conversation, and now, thanks to Kevin Smith's show, we have a better idea of what Zack Snyder envisioned prior to reshoots.
With rumors and leaks swirling all around regarding what went wrong, Some say Zack Snyder is to blame for his polarizing vision for the DC universe, some say Joss Whedon ruined what would have been an epic conclusion to Snyder's Superman trilogy, and still others point to WB executives, claiming they were more interested in getting their bonus check than giving Justice League the time it needed by delaying the release date.
It will likely be years before all the details are sorted out, and even longer before the finger pointing stops, but in the meantime, some details have been revealed about exactly what changed between Zack Snyder's original footage and Joss Whedon's final cut. It's hard to validate many of these scenes, and a number of suspect "leaks" have been posted around the internet, including a list of changed and deleted scenes that Kevin Smith read off on his show, Fatman on Batman. While they originally popped up on the /r/DCEULeaks page on Reddit and were soon deleted, they have since been posted elsewhere, and Smith actually had an audience member who was able to validate that the version in question, which purportedly was screened earlier this year, was the same as one of the versions she had seen test screened (with the other being a version much closer to the theatrical release).
On a high level, the plot is relatively similar, but Joss Whedon's final cut removed a number of worldbuilding elements, especially relating to Apokolips and Darkseid. As was rumored before the release of the movie, the original version of Justice League was supposed to introduce Darkseid to the DCEU and set up a sequel in Justice League 2, but the ending was reshot, removing that element entirely and making it a standalone story.
Originally, after failing on Earth, Steppenwolf returns to Apokolips where Darkseid kills him and say's he intends to meet Earth's "Kryptonian," but instead the theatrical cut incorporates the subplot that parademons smell fear through Joss Whedon's opening Batman scene, which then ends up being Steppenwolf's demise as he begins to fear the united Justice League with Superman and his own soldiers attack him and they disappear into a Boom Tube.
There's also a number of extended scenes that flesh out backstory for The Flash, Cyborg, and Aquaman - including a significant amount of Atlantis worldbuilding - and tie back to the events of Man of Steel and Batman v Superman, including references to the Knightmare sequence and the incorporation of The Codex in Superman's resurrection (as we predicted prior to the movie's release). Batman is also remorseful over his darker days and treatment of Superman, behaving in a much less jokey way, even going on a suicide mission at the end, believing he needs to sacrifice himself to atone for becoming a murderer and trying to kill the Man of Steel.
In addition to that, there's a number of cut characters, like Iris West, Deathstroke, and Lex Luthor, with the latter two still appearing in an end credits scene. This leak also corroborates the previously revealed information that another post-credits scene featuring Green Lanterns had been scrapped early on.
It's apparent Warner Bros. screened multiple versions of the film to see what registered with audiences better, and Joss Whedon's added humor obviously made a bigger impact in the end. While test screenings aren't always an accurate measure of how well a film is ultimately received, if this newly revealed version of the movie had been the one to arrive in theaters, it likely would have seen a similar reception to Batman v Superman, considering it ties back to the events of that film so cleanly.
Whether this is the "Snyder Cut" fans have been petitioning for, or an early version of Joss Whedon's movie that actually adhered to Snyder's vision, as promised, isn't clear, but considering how well it meshes with the themes of the previous two movies, Snyder's sensibilities are a lot more apparent here than they were in the theatrical cut. There's been a lot of conflicting information about whether or not a Snyder cut even exists, with some insiders saying it would never be released, while others say it was mostly complete, and the fact that this was being shown at test screenings (with incomplete effects) definitely leans toward the latter.
Source: Fatman on Batman