Batman v Superman is not the 30% it is on Rotten Tomatoes, but it has some big issues. I can put them down to three major issues, two of which I was expecting, one I didn't. I won't spoil anything story related, but I have some opinions and if trying to go fresh as possible, best to not read below.
-They are rushing the next movies to build up to Justice League, so they shove everything they needed into this one movie to set up everything. Wonder Woman was handled well, I was fine with her setup. Same for Batman. They hint at some things we kinda already saw in trailers, but it would lead into the Batman movie that could be good. The problem is, they are not only trying to setup plot elements of the Justice League movie, but they blatantly show us Cyborg, Flash, and Aquaman. And not in a quick tease, but like there is a moment where we get good long looks at all three. It stops the movie for no reason. They teased them perfectly before, but then it's "NO, JUST LOOK AT THEM. THEY ARE COMING". Them doing this adds like 15 minutes to the movie at the very least for both of the future movies setup.
-Lex Luthor is awful. Eisenberg was a miserable choice. You know what he should've been cast as? The Riddler! He might've made for the best Riddler in the Batman movie. In this movie, Lex is not intimidating, he's not scary, he isn't threatening really beyond some actions he takes in this. I don't know what they were thinking. It's a shame that Kevin Spacey was wasted as Lex in Superman Returns. You have Kevin Spacey here as Lex, you got yourself an incredible Lex Luthor.
-Batman kills people. This isn't like in the recent Arkham Knight game where Batman driving around in his car that stuns guys as he hits them and they want to play it off that they are fine. I'm fine with a brutal Batman. No, he kills people on purpose. It's such a problem, because I think Batman in this is great. When we get that Batman movie, I'm so excited, because this is a great Batman. Affleck nails it, the suit looks great, there's some serious story they tease about what happened in his past that could make for an amazing movie. But he kills people and it's never in a Man of Steel kind of why, where it's a huge struggle for Superman to not kill Zod, no he just doesn't care if people die in this. Which makes one great fight scene later kind of a problem, because if Batman will kill people, there's no reason for him to just not turn into the Punisher. It's something that I think they could do away with in the Batman movie by just ignoring the kills in BvS. It makes me wonder what the R rated cut of this will be, because it could just make Batman even more violent.