Kinnikuman at first started off as a gag manga parodying Ultraman at the time, but over the course of the series, the Ultraman elements will be dropped such as the techniques that are parody versions of the main characters, the ability to turn into a giant and even the Kaijus stopping appearing. In exchange it becomes a wrestling based fighting shonen where Chojin (basically Supermen) fight each other for glory or for the sake of the universe.
There are a huge selection of Chojin types each with their own believe system like the Justice Chojin fighting for the safety of those without powers and the Devil Chojin to rule over everybody and aren't afraid to fight dirty.
Like in One Piece, there is a huge variety of characters actually send by fan who read Shonen Jump at the time. Some of the designs get merged into a more fitting character but the authors (it is worked by the duo Yoshinori Nakai and Takashi Shimada, known as Yudetamago) do credit the kids or adults who send them the designs. Some of these characters become protagonists or even major antagonists for example Robin Mask one of the fan favorites.
What makes it so great is that the character designs are not just for show but actually affect the fighting style of each Chojin. Each using their bodyshape and size to full effect coming with unique strength and weakness. An example is Prisman, a Chojin made out of glass, who uses his sharp edges to cut. His opponents can hurt themselves when they attack him with karate chops. And he can absorb sunlight releasing a dangerous beam of light.
Another example would be Planetman who is made out of all the planets in our Solar System and has the ring of Saturn as a sauzer to cut things or freeze opponents with his legs which are representing the planets furthest away from the sun.
Kinnikuman is also one of few shonen that grew up with its audience as after the main run concluded, but its sequel Kinnikuman Nisei and its 2011 continuation were released on a mature oriented magazine where they aren't restricted from showing things inappropriate for a younger audience.
There is also the theme of friendship in the show with the use of friendship power which is usually associated with "random power out of nowhere" or "deus ex machina", basically bullshit powers. However, Kinnikuman is one of few shows that uses friendship power appropriately as it has conditions, weaknesses and can be even harvest by the enemy.
The 2011 continuation of Kinnikuman also came with improvements addressing many issues of the original run as well. Examples as such improving the art quality, showing a few of the improvements and changes that will appear in the sequel, adding advancements made by Shonen that came after like visually highlighting a power up called the Burning Inner Strength, giving more screentime to characters that didn't get to do a lot as well as getting even more out of the designs of older characters and expanding more on the lore of the world such as the good gods who only appeared as faces instead as characters unlike the evil gods.
Meaning unlike many other shonen who continued longer than needed, Kinnikuman's 2011 continuation doesn't devalue what came before it nor does it feel like a cashgrab as one can see the love and care previously put on the series in it.
As for the characters, Kinnikuman also known as Prince Suguru Kinniku is one of few corruptible main characters. He doesn't have an evil demon or anything but he is a selfish adult. At first he appears to be very unbearable and unreliable, but from time to time he shows some signs of humanity in them needed for a shonen protagonist you look up to. Over the course of the series, while he is scared and doesn't want to participate in certain events like fighting major villains, he makes up his mind and fights for those he loves developing courage and discovering the hidden power Burning Inner Strength. He would cast his old techniques for ones taught by the former Hawaian Champion Prince Kameha and eventually developing his own ones. Over time gaining the respect of all the people who hated him which is... pretty much every living soul but his parents.
Meat is Kinnikuman's side kick or rather servant. He will be his closest friend. While not the most powerful fighter and rarely even participates in combat, he is often involved in some shape or form in Kinnikuman's business.
Terryman is a chojin from America Texas who thanks to Kinnikuman changed his world of view and saw the greater things in live over the things he felt he needs to do. Often he is the person Suguru can rely on the most.
Robin Mask was the Chojin Champion of the start of the story and one of the most respected character and a real rolemodel. Over the course of the series he would go through major character arcs and is one of the most well developed ones in the show.
There are many other characters but listing them all and explaining something about them would take forever.
A criticism can be seen as the shift from parody to a completely different product. So if you don't like both of those and the shift isn't something you like, it can be a turn off. But the tonal shift happens gradually enough to feel natural.
Another thing is that this manga is victim of some changes in the way things are viewed now. What used to be for kids back in the day is even intolerable for some adults as there are some very random insanely brutal scenes or a character based on the history of something said country doesn't want to be reminded off.
Continuing Kinnikuman despite having a concluded sequel can be off putting for some people who know that the characters say saw around a decade ago in the latest story are fine. But 2011 continuation fills a few plotholes and creates a set up to better transition to the new generation.
Death can appear as meaningless since like in Dragon Ball characters can come back to life. 2011 version fixes that issue too by expanding on the story of the Chojin Graveyard and its mechanics as well as a way to remove it. Meaning after that and in the new Generation continuation, death will be permanent. But even before that, there were very strict conditions that made it impossible for any character to come back at any point. It wasn't as easy as in more modern shonen.
And the character designs especially the main character can be off putting.
The manga is also ongoing meaning the show may not end great like the original run which ended during the Scramble for the Throne arc.
In conclusion, Kinnikuman is a great shonen and a must read for Shonen fans especially who love wrestling.
