Prior to its theatrical release, there had been a lot of positive feedback from test audiences. Then reviews from critics came out, and they were pretty negative.
While there are definitely some people that genuinely do not like the movie, I believe that there are people that do not want to like the movie. Those people are predominantly extremely loyal Marvel fans. The thing is, that if this movie came out four years ago, there would likely be universal praise for it. Much like the near universal praise for the Dark Knight trilogy.
Now that we live in a superhero movie era, dominate by Marvel, DC is at a major disadvantage. Marvel has spent the past eight years developing a solid cinematic universe, while DC has been largely quiet. Because DC is so late, they have lost a lot of potential fans to Marvel. Marvel's fans are competitive and strongly feel that Marvel's movies are better than DC's. That is why they like to scoff at DC's developing cinematic franchise.
Allow me to take the time to counter the arguments that Batman vs Superman was a bad movie.
Batman vs Superman wasn't focused enough on Batman and Superman fighting.
The whole movie was about Batman gearing up for war with Superman. Everything he did during the beginning involved tracking down leads to get the kryptonite. Meanwhile, Luthor was doing his work behind the scenes setting up the two titans to face off against each other.
Some people criticize the dream sequences. Given the fact that Batman is Batman, it is very likely that he would be having dark and disturbing dreams. The dream where the Flash visits him is particularly perplexing. The Flash appeared to be trapped by something (possibly a Brainiac machine thing?) and told Batman that he had always been right about "him" (we assume Superman). Whatever that was supposed to mean, it played a role in the end. Batman told Wonder Woman that he had a feeling that the other superheroes needed to team up because he had a feeling (aka the dream with the Flash). Ultimately those scenes played an intricate role in developing the plot.
There wasn't enough action in the movie, it was too slow.
For the average viewer that isn't too interested in the comics, the beginning might have been a bit slower. But it was filled with interesting facts about the characters and comic book easter eggs. Then, there was about a good 30-40 minutes of fight scenes throughout the movie. If you think about it, a lot of stuff went down in that movie.
The fight between Batman and Superman could have been avoided.
It will be interesting to see how Marvel's Civil War will play out, because it is likely that same claim could be made against that as well. But then there would have been no movie. Additionally, Batman was so motivated to destroy Superman, that it would have been hard for Superman to talk it out with him. Remember, an entire Wayne building fell down and Batman was haunted by dreams of powerlessness.
Batman vs Superman is not on par with the Avengers.
I would argue that it is. But it is not intended to be. Remember, this didn't have the entire Justice League in it. That movie is coming out next year. It's more like one of those lesser movies in the superhero cinematic universe, like the Thor, Captain America, and Iron Man movies. The bigger DC team-up movies are yet to come.
The movie did not follow the comics.
This has been a common criticism among DC comic book fans. One of the problems is that there are so many different comic book versions of these superheroes, that it can be tough to incorporate most of them into the move. Yes Jimmy Olson and Mercy Graves were killed off very very early on. Yes, Superman killed someone in the Man of Steel and Batman killed many people in this movie. But there were a lot of elements there that came from the comics. For example, Superman's death at the hands of Doomsday is similar to some of the comics from the 90s.
Ultimately, I thoroughly enjoyed Batman vs Superman. I thought the actors were phenomenal and that the entire movie was visually stunning. The special effects were really done well and the plot of the movie was also great. This is a very solid start to what will likely be a spectacular franchise. Personally, am looking forward to the future DC movies to come.