(THIS ARTICLE CONTAINS MILD SPOILERS FOR THE MOVIE. IF YOU HAVEN'T WATCHED THE MOVIE YET, YOU WILL REGRET READING THIS)
1) When Bruce Wayne visits his parents' grave house, he notices blood seeping out of a ridge in his mother's grave stone. He touches it to see what it is. The quantity increases. Suddenly a giant BAT-like creature breaks through the stones and attacks him. The creature looks highly similar to the villain Man-Bat (Dr. Langstrom)
2) In Batman's KNIGHTMARE sequence, we get our first clear look at how the Para-Demons look like in close up. When Batman is captured by the soldiers of Superman, a Para-Demon comes and punch Batman in the face to make him unconscious. I found the following image on Imgur and its the real one.
3)BATMAN KILLS PEOPLE!!. Yes, he kills some of Superman's soldiers in the Knightmare sequence.
4) There is an awesome reference to Injustice story-line. When Batman is captive in the underground compartment, Superman comes and cuts three men with his lasers. Also in the end, he sends his hand right through Batman for not saving Lois Lane. This is a reference to how Superman kills the Joker also for killing Lois along with bombing the whole of Metropolis
5) Right after the Knightmare sequence ends Bruce wakes up only to find himself in another dream. In this, Flash tries to rip through time to warn Bruce about something. The following words are what Flash says to him.
LISTEN TO ME RIGHT NOW
SHES THE KEY
AM I TO SOON
YOU WERE RIGHT ABOUT HIM
YOU WERE ALWAYS RIGHT ABOUT HIM
YOU HAVE TO FIND US BRUCE
YOU HAVE TO FIND US!
6) Now coming to the other cameos, we see Cyborg, Flash(again) and Aquaman when Wonder-Woman views the folders in Lex Luthor's files. We see Barry Allen in a market when a thief threatens the person the counter. He immediately zaps and hit's the thief and comes back to his places. We see Aquaman coming out from a broken ship with his spear and attacking the drone sent to find him. Finally we see Dr.Stone in a lab experimenting on his son's burned body. After a few failed tests, we see him placing a mother-box(Darksied's belonging) near him. It activates on it's own and start turning into limbs and attaching to Victor's body. We can hear him screaming with pain.
7) KRYPTONITE SPEAR!!!
8) Doomsday is made of Zod's body and Lex Luthor's Blood.
9) When Doomsday first merges out of his cocoon he somewhat looks like an ogre from The Hobbit. He is all gooey and gray!
10) Doomsday becomes more powerful every time he is attacked.
11) In his first transformation, he sheds his skin. From inside emerges a dry bony skin, like we saw in the trailer.
12) He kinda has a second transformation, but the bones only grow on his fists.
13) SUPERMAN DIES!!! He stabs Doomsday with the Kryptonite spear. But he is also stabbed by one of doomsday's bones.
14) At the ending of the movie, we can see Bruce and Diana talking to each other. Bruce says that they have to find the other meta-humans and that they have to FIGHT!! When Wonder-Woman asks why, he simply says that he felt like that.
15) When Batman meets Lex Luthor in the prison, Lex says that their time is up and that "HE" is coming. After Batman leaves he starts singing DING DONG DING DING DONG DING DONG DONG....
16) The last one, After burying superman's coffin in the farm, Lois drops some mud on the coffin. After everyone leaves the place, the mud starts to float. Just like the ice does in Man of Steel, when superman flies for the first time.
Alright, you are at the end of this article. I might have missed some. If you know more, please put them in the comments sections.