SLIGHT SPOILERS BELOW...
...But nothing you couldn't guess from the trailers.
Batman v Superman: Dawn of Justice is not a good movie. There are actually a lot of things that go into this, and there have already been great pieces written on how it just really doesn't work on any number of levels (it's overly-complicated yet very stupid, it's aesthetically ugly to watch, it's poorly-paced and badly edited...for a start), but the Gordian Knot at the center of this cinematic catastrophe is actually a fairly easy one to pick apart.
See, DC and Warner Bros. decided that they wanted to use this not-really-a-sequel to 2013's misguided Man of Steel to launch an Avengers-style superhero movie universe, but they also decided that they wanted to do this by mashing together adaptations of the best-known Batman and Superman books of all time. Unfortunately, "The Dark Knight Returns" and "The Death of Superman" (which, make no mistake, are what this mess was birthed from) are both stories meant to bring a literal END to their characters. One was a conscious deconstruction of both Batman and Superman as heroic icons, and the other was a nakedly obvious sales stunt, and even if there had been a real thematic exploration in the film and its execution had been immaculate, it still would have been a very awkward way to start the DC Extended Universe.
But with the movie being...well, what it is, WB has built their house on a festering cemetery without even bothering to move the headstones, let alone the graves.
Dawn of Justice picks up during the Battle of Metropolis from Man of Steel, and if you thought that film was already uncomfortable in its 9/11 allegory and heroically-ignored civilian casualties, just wait until you see how BvS doubles down on both of those things. This is part of the film's ludicrously convoluted plot to pit an aging Batman against Metropolis' alien visitor, but it also shows just how horrifically writers David Goyer and Chris Terrior and director Zack Snyder misunderstand why people didn't respond well to their carnage last time. Yes, people in BvS are concerned about Superman being responsible for death and damage, but nearly all of them turn out to be manipulated into their distrust by Lex Luthor (we'll get to him, but he's terrible), so it feels empty. Almost like the movie is saying "Shut up - YOU SHOULD BE THANKING HIM FROM YOUR PILES OF RUBBLE!" like some Super Saiyan version of Ayn Rand.
Even worse, Clark never acknowledges any responsibility, or shows any lingering guilt about his near-cataclysmic actions. Hell, early in the movie, there's a scene where Lois Lane expresses similar concerns about his recent actions only to have him brush those off with a (and I'm paraphrasing, but not much) "Hey, my woman was in danger - now let's bang."
Maybe there's something missing that could salvage Superman as a character in these films (the R-rated "Ultimate Cut" that releases later this year on blu-ray is reportedly 3 hours long), but as it stands, Superman isn't just a bad version of Superman, he's barely a character in what is supposedly a sequel to his own origin story. Henry Cavill gets even less chance to show a glimmer of charm or warmth than he did in Man of Steel, as its Ben Affleck's Batman who is far more the star of this film. And while Affleck throws his considerable charisma and acting prowess into the role to good results (and has dynamite chemistry with Jeremy Irons' Alfred), his character is no better-written. In fact, while he gets more screen time and something resembling an arc (if a thuddingly stupid one), changes made to Batman's moral code actually break him just as badly as Snyder has broken Superman.
Luckily, Gal Gadot's Wonder Woman emerges more-or-less unscathed...partly because she's in the movie for a grand total of 10 of its whopping 151 minutes. Lois Lane barely fares better (having literally nothing to do but chase a mystery that Batman has already solved in between getting herself captured), and the way the film casually kills off other fan-favorite DC characters feels almost like Snyder is actively trolling the "in-the-know" audience at this point. Which is odd, because the film is going to be damn-near impenetrable at times to anyone who can't piece together the hints about otherworldly conquerors that get thrown into the mix in the name of sequel-teasing and world-building - in the most awkward and obtuse way possible, no less.
But the crown jewel of awkward, obtuse, stupid writing, and next-level trolling comes in the form of Lex Luthor. Jesse Eisenberg's take on the character plays like someone dared him to go even bigger and louder than Kevin Spacey did in 2006's Superman Returns while employing none of of the watchable charisma or occasional threatening presence, all while reading what sounds like a DC version of Drunken Mad Libs. Luthor here is by turns an anti-alien xenophobe, a man with a fixation on killing God, an abused son, or the evil genius who saw the coming cosmic threat that requires the creation of the Justice League, depending on what scene he's in. It's like the filmmakers wanted the Joker again but decided to give him a different name because this was still ostensibly a Superman story.
As the driving force behind the entire plot, Luthor is inflicted on the audience for a grueling two hours until the World's Finest can finally throw down and punch each other before the inevitable team-up, and for all that Zack Snyder is usually an action virtuoso, neither of the two big would-be set piece are even visually arresting, much less engrossing from a meaningful story or character standpoint. Even an obvious late-in-the-game attempt at an emotional punch is hamstrung by the fact that the film hasn't done the necessary legwork to really invest us in these characters and their relationships (even assuming you forgot about certain characters' required attendance in the upcoming Justice League movies).
I'm sure BvS will find an audience, but I can't be counted among its fans. Aside from being an under-cooked and misguided mess from a conceptual level and poorly-constructed throughout, the film seems actively ashamed of its iconic and aspirational characters. Bringing the gods of Detective Comics down to our own base level, Dawn of Justice feels less like an exciting beginning than the start of a long, dark, cold night.