The rumors have to do with how Superman will possibly be treated in the movie, as well as Batman and Wonder Woman's beginnings.
Regarding Superman, Batman-On-Film is stating the character will be treated badly in the movie - an "anti-Superman theme" - with people actually protesting against him.
For Batman, Bad Ass Digest is stating that the character will be in his 50s, and that Batman has existed for 30 years. It's said in those 30 years, Batman is still is considered an "urban legend, a creature of the night," and that he has never had his picture taken. The Batcave is also said to include a memorial centering around a ragged Robin costume.
For Wonder Woman, it's also said she has been in operation for a number of years preceding the movie and just as secretive.
"Batman v Superman: Dawn of Justice" has a May 6, 2016 release starring Ben Affleck as Batman, Henry Cavill as Superman, Gal Gadot as Wonder Woman, Amy Adams as Lois Lane, Laurence Fishburne as Perry White, Diane Lane as Martha Kent, Jeremy Irons as Alfred, Jesse Eisenberg as Lex Luthor, Ray Fisher as Cyborg with Callan Mulvey, Holly Hunter and Tao Okamoto in new character roles for the film. Justice League is to follow directed by Zack Snyder as well.
For more news on the "Man of Steel" and related movies head on over to the Cosmic Book News Superman movie hub.