The sad truth is that they had all of the right tools to build an epic, they had titans, walls even the 3D manoeuvre gear but instead they watered the source material down to a hollow shell of what it once was, the trailers showed all of the promise of the original franchise but sadly that is all we got to see of the anime being brought to life and that made the movie even worse.
Like the anime the movie is set in a post-apocalyptic world where what's left of humanity has been confined inside three huge walls. The walls were built to keep humanoid creatures known as Titans from getting in and for one hundred years its worked. This all goes to hell when a Titan that bigger than the wall appears and smashes a giant hole through the wall and lets in all of the titans and then death and war ensues for the foreseeable future.
The film follows three teenagers Eren, Mikasa and Armin, they see witness the death and destruction that titans bring and two years later they join the army so they can fight titans and reclaim their home. In a massive battle Eren get eaten by a titan and later rips his way out by becoming a titan himself and begins decimating other titans around him and giving humanity a new hope in the war with the titans.
So where did they go wrong? Well to begin with they chewed up the original storyline of the manga (which was a piece written genius) and spat out something vaguely similar. One of the key defining moments in the anime is when Eren is forced to see his own mother being devoured by a titan, this then sets Eren on his path to take revenge and become a titan himself this character element is what makes you the audience emotionally invested in Eren's story, and for some stupid reason this never happens in the movie, it just doesn't happen and instead Eren's drive to kill titans is because "he wants to see the world outside of the walls" which is really weak reason and sounds stupid when you shout it out of the titans.
As character screw ups go, they really did a number on Mikasa, being Eren's adopted sister and love interest, her character was vital in the manga and anime, she also was one of the most badass characters in the anime and brought intensity to the story. In the movie for some reason they thought 'pretend killing' her off for two years was a good idea and just leaving Eren and Armin to join the army brought the movie down seeing that her character kept Eren on the straight and narrow and forced him to become better person and focus his anger. In the last twenty minutes of the film they bring Mikasa back and apparently she has been training in secret with a veteran titan killer, this development brought the movie to screeching halt, because the movie failed to establish there were soldiers fighting titans or they had veterans. Mikasa was integral to the storyline and she was barely in it.
Armin barely got any screen time, which was a shame because he was the only character they got right in the movie, he was the strategist that came with masterful plans to kill titans but none of this was used in the film. The titan killing veteran Captain Shikishima was a pale imitation of Captain Levi the most beloved character in the original franchise, simply because he was just awesome on the battlefield and had calm attitude in combat, once again none of these elements were in the film and the character was wasted.
The movie score was up and down like a yo-yo, at first the soundtrack was similar to the anime but then suddenly Godzilla music would awkwardly interject and completely destroy the atmosphere the scene. The soundtrack of a film is a foundation that you need to get right otherwise the film fails, the soundtrack creates atmosphere and helps build emotion which this score did not do (at all).
The setting and location of the film is actually one of the few elements I was happy about, it actually looked like the anime and manga. The giant walls and crumbling houses really set the scene and showed you what conditions were like within the walls and revealed why they needed to defeat the titans.
Another positive note was special effects in the movie, everybody expected the titans to look like monsters from a b-movie, I actually thought it gave the movie some charm and a unique quality that it drastically needed. Sadly as the movie progressed the effects wore thin and just became annoying to watch, the gore helped lighten the annoyance and made the titan scenes fun to watch which was not a good thing, mainly because blood and gore is more of an embellishment not a core element that drives the movie forward. I believe they may have blew most of the effects budget on Eren transforming into a titan, because its the only scene in the movie that's worth sticking around for.
As a whole, what made this movie such a disappointment was how much hype there was for it, I expected the movie to be adaptation of the anime and that there would be some changes but straying from the original story that made the franchise what it is was the what killed it. As a viewer you have no emotional investment, even if you haven't heard of the anime or read the manga the plot was filled with so many holes you wouldn't have a clear idea of the actual story and what the character's goals are.
For perspective the video below is an example of the fantastic source material the movie makers squandered.