So picture this: You're Japan's greatest hero, your power is fading fast because of the terrible injuries you sustained while fighting your archnemesis while you were at full power. You know he's going to return and you need somebody to pass your powers on to so that they can stop him. Do you choose:

A) One of the top 3 best students in the best hero training school in the country with 17 years of practice handling a high difficulty ability that makes him nearly invincible, the attitude and drive of a professional and a recommendation and combat training from your trusted sidekick who can see the future
OR
B) A kid who threw his bag at a villain, ran into danger with no plan whatsoever that you met that day but he said he wanted to be a hero and cried about it despite not even putting in the effort to be physically fit.

If you answered B you have what it takes to be the number 1 hero in Japan according to this manga.

In case you were still unsure from that one example the characters in this are as flat as the paper they're drawn on and act in ways that are beyond my mere mortal understanding.

We have our main character Midoriya "Deku" Izuku a boy who was born without a quirk like around 10% of the total population in the HeroAca world. Instead of choosing to get a normal job he decided that because of a YouTube video he watched when he was like 5 he wanted to be a hero, not just any hero mind you considering all kinds of heroes exist in this world but a hero like All Might, the punchiest , liftiest hero in all of Japan. He plans to accomplish this through a strict regimen of doodling in notebooks while whispering to himself and crying until one of the strongest abilities in the story falls into his lap along with a free mentor character. The story also wants you to believe that Izuku is a very smart boy but it took him until much later in the story to realize that kicking was a thing. He moves the story along by doing nothing at all until the next arc finds him.
The rest of the cast are just as bad going from the background fodder who have like 1 character trait (read literally every U.A student that isn't a main character) to Endeavor the man who mentally scarred every single member of his family but it's cool now because he had an awesome fight scene.
The best character is Shigaraki Tomura the villain that is being built up as Deku's nemesis since he is the only person in the story who learns and develops as it goes on and has a clear motivation.

The setting of HeroAca is as baffling as its cast. Imagine if everyone in the world suddenly got superpowers, for good or bad society as we currently know it is done. Yet in this world the only difference is that there is a new job called "hero". That's it. No different infrastructure to accommodate the different kinds of people that have evolved, no tension from people with bird heads riding the bus with you, everybody is fine with needing a license to use their quirks except for the most extreme villains or unorganized street thugs.
The majority of the story takes place in U.A the hero school the main characters attend yet somehow we've only managed to see 3 senior students. Where the hell is Class 2A? Nobody knows.

The art is great and Horikoshi is a master of character design. The fight scenes are well drawn and if the characters had as much personality as their designs promised this manga would have been excellent.

The story is bland shonen fare and anybody that tells you " but it does shonen better" is wrong. If you like shonen you'll like this ( at least for a while till the inconsistencies and lack of depth get you). There are no surprises, no plot twists worth mentioning. The students go to school and do school stuff, villains do their thing, they fight, students win. Rinse and repeat with differing scale and a different person defeating the villain each arc and congratulations you have the story so far. I can't wait for the arc where the heroes lose, everyone acts sad for a bit and then they come back and win.

Overall it's a mediocre manga with great art, terrible characters with excellent designs and a world with as much depth as that generic fantasy world that every bad Isekai is set in.