Of course, not everyone in country dislikes what they have seen so far. And I do think the movie should do well, just based on the built-in fan base and the ability of the mainstream Japanese media to create hype.
However! There have been several reoccurring complaints about the movie, which typically pop up online in Japan. Complaints online? You say. IMPOSSIBLE. Yes, I know, I know. But, these complaints -- however picky they are -- should provide some insight into some of the scepticism towards the upcoming film adaptation.
Manga fans in Japan are just are protective of the original works just as, say, gamers in the West are about Hollywood movie versions of their favourite games. The reason is simple: They have been burned too many times. Sure, for every good adaptation like the Rurouni Kenshin movies, there are plenty more awful ones churned out by Japanese studios.
Fans want the original work respected! That's certainly understandable. Yet, they're different mediums with different creators -- which is where the trouble often arises. It's no surprise that the Attack on Titan movie seems to have made changes to the original manga. For example, the character Levi Ackerman is apparently not in the film. Instead, a totally new character named Shikishima will appear. Stuff like this makes people nervous.
There are other qualms. Some are very picky. For example, some don't like the movie's full title, which is Attack on Titan: End of the World. In Japanese, the subtitle is written as "Endo obu za Waarudo" (エンドオブザワールド), and some felt like a proper Japanese language subtitle would be better. I kind of agree with that! But whatever.
Then, there are cast issues. In the manga and the anime, the characters are white, with most having German or European sounding names. In the movie, they're Asian. Online in Japan, many complaints against the film indicate that some don't want to see Japanese actors playing characters who are not supposed to be Japanese. It seems odd, they say.
And then, there's a somewhat of a hot-button issue. Online, there have been complaints about the actor who plays Mikasa, a character with a popular Japanese name and Asian heritage. In the movie, Mikasa is played by Kiko Mizuhara, who is half-American and half-Korean (her mother is a Zainichi Korean). And while she was born in the States, she did grow up in Japan. But since she's not technically a Japanese citizen, some people online in Japan (certainly, not all!) find it curious that she'd play the manga's main Attack on Titan character that clearly seems to have Japanese heritage. While this has way different cultural baggage and there is a heck of a lot to unpack, in the West, a vaguely similar argument is sometimes made when Hollywood hires Asian actors to play characters of different nationalities, whether that's having Chinese actors play Japanese characters or vice-versa. Mizuhara is currently a popular model and celebrity in Japan. She grew up in Japan. Of course, she speaks Japanese. And it seems the film's producers thought she best fits the part, regardless of her nationality.
Via internet forum 2ch and numerous other Japanese blogs (here, here, here, here, here, and here), below you can see a cross-section of comments in no particular order. Some comments are directed at the above posters, while others are the trailers.
Like anything you see online, don't think these opinions as indicative of all the 120 million or so people who live in Japan. However, the comments might provide an interesting look at how some people view the upcoming film. And, yes, to be completely fair to the filmmakers and the actors, save your final opinion until you've seen the completed work.
With that being said, here goes:
"The buildings [in the background] look like Japanese apartments... Yet, they're still using the name [Attack on Titan] without any changes... Um..."
"Even though their names are for foreigners, their faces are Japanese..."
"Everyone is Asian. lol"
"I think if you are going to make changes to the original work, then it's better just to make something completely original."
"It looks rather shoddy."
"I think it looks interesting."
"I'm more worried about the actors than I am the CG."
"If you're going to make it real, then Mikasa's scarf looks like it would get in the way."
"If you changed their names, you wouldn't know who is who."
"Eren (Asian person)."
"Why did they set it in the modern day. hahaha"
"This looks more like expensive cosplay."
"The time period is strange. Fight with regular weapons, and if they used a modern military, they wouldn't lose."
"In the original work, the only Asian-like one is Mikasa."
"It seems like something something to be ashamed of."
"It makes feel uncomfortable that all the characters look Japanese."
"This looks like a middle-school or a high-school play."
"Why is the character with Japanese heritage the only one with the face of a foreigner?"
"Why isn't a Japanese person playing Mikasa?"
"They really should have stuck to the original story."
"This is totally embarrassing. Please release it only in Japan."
"We're really not going to know until we see the movie."
"Even though the character of Mikasa has a popular Japanese name, she's being played by a half-American and half-Korean actress."
"This should have have been made in Japan."
"This always happens."
"Why doesn't Hollywood do a live-action version?"
"Compared to the Hollywood version of Dragon Ball, it's fine."
"Well, if you are uneasy about it, don't watch it."
Attack on Titan: End of the World will be out later this year in Japan.