Currently, Shingeki no Kyojin (or Attack on Titan) was one of the greatest manga series I have ever read. I believe a lot of the fandom is very disappointed with the current ending, the ending feels very rushed to me and to a lot of the fandom. The ending overall feels very thematically different, giving a lot of fans tonal whiplash. Now, this ending doesn’t ruin the whole story, but it does feel somewhat off.

One example of this would be that some panels almost looked like something out of a comedy series. The ending message was also quite disturbing, a little girl or "Founder Ymir" was truly "in love" with her pedophilic captor who had also raped her. Something just feels off about romanticizing something like that. It was quite literally called "love." All of a sudden the characters motives changed (mainly Eren Jaeger's goals) everything in the story now feels rather pointless. Characters like Mikasa Ackerman and Historia Reiss now feel sort of wasted, everything feels far too different from the story that we've come to know. 

Another disturbing message is how Eren simply did everything for the fact he "doesn't know." Another unfitting thing is; Eren in the previous chapters told Mikasa to "forget" about him, yet in this chapter his entire character contradicts himself by forcing Mikasa to only love him and to be obsessed with him for years. This doesn't feel thematically correct with everything that has occurred within Attack on Titan, the ending very feels out of place. The romance itself could fit perfectly if Eren was a tad bit less creepy about it. It's just odd he wants her to love him for years even after he dies, he doesn't even want her to find another man. That seems more like obsession which didn't sit right with some. Eren isn't a "simp" but it seems rather strange he wants Mikasa to only love him and no one else. 

It felt like the ending gave the rest of the characters plot armor, which is quite strange considering Isayama had zero problem killing characters a few chapters ago. (Hanji for an example.) The main victim of this ending was Eren, who loved his mother dearly. But in a way he was a cause for his own mother's death, somehow he led the Titan to his mother causing her death. Though others mention how Ymir caused it and orchestrated it. So this part is rather unknown.

A very large portion of the fandom from what I've witnessed believes that Kodansha or Isayama's Editor has heavily influenced the ending to perhaps fit a more "shounen" type of ending. In Japan the studio and editors typically have more power than the author himself. Example, when the editor changed Sasha's death (which was originally going to happen when Sasha battled the titan in season 2) not only that, but Isayama did not even write the final note to the reader, it was the editor. While I don't entirely believe it, it does have some evidence to prove it, but in recent interviews Isayama has debunked this. The ending didn't seem "happy" but nor did it seem dark enough, it was a little too ambiguous leaving many important questions up for debate. Though this is mere speculation. Now, I cannot speak for the whole fandom, but I've seen hundreds and hundreds of fans (including myself) express how disappointed in how characters were simply wasted and along with how the story did a complete 180. 

The overall tone doesn't fit and the final panel makes zero sense to many, as Grisha mentions to Eren how he is "free." Which would not make sense as Grisha quite literally forced Eren into being a Titan from what it seemed, but the last few chapters (mainly 137-139) suddenly got convoluted causing the story to be incredibly confusing. There are still several large unanswered questions "What happened to the Titan Parasite?" "Did the Titan shifters in the paths die? Because it seems they can survive death." "Why didn't Eren control the Titans he made?" and so many more.

 Along with that the Alliance easily beats our antagonist, throughout the whole story of Attack on Titan the power of "friendship" has never prevailed, so why is it different here? Why does the trope "Talk no Justsu" work? There were several other issues, but the main problem was that the story's conclusion felt very unsatisfying and too tonally different from the story the fandom and I have grown up with. This by no reason means you should go and harass the author Hajime Isayama. The goal is to perhaps allow MAPPA to elaborate more on the ending. My personal hope is to get an anime original ending where Reiner is more of a "Helos" but that's just my personal hope!