Many rumors swirl around the WB/DC movies, especially one as highly anticipated as Justice League, but rather than focusing on rumors, it is more concrete to look at the official trailers that were released over the past year and compare that to the theatrical version of the film. The video below contains all the shots and dialogue that were present in trailers but absent from the finished film. This is based on my memory having seen it twice, so although I think it's quite accurate and comprehensive, I can't guarantee that I caught everything.
One thing to keep in mind is that some of these cuts were just specific shots from a scene that was in the film. But other cuts seem to imply that whole scenes were removed. The scene-level ones that I miss the most are Victor Stone before the accident, Barry Allen with the glass, and Bruce talking about his dream.
In addition to the trailer content that was cut, I personally was also disappointed that what Ciaran Hinds described about Steppenwolf wanting to get out from under the oppressive rule of his nephew, Darkseid, was absent from the film, too.
Here's hoping for an extended cut on the blu-ray!