The forth install of JoJo's Bizarre Adventure is Diamond Is Unbreakable, a story which follows the forgotten son of Joseph Joestar, Josuke Higashikata. He and his friends trek across the town of Morioh, engaging in a diverse number of misadventures and ultimately tracking down Yoshikage Kira, a deranged serial killer who had murdered their friend.
As entertaining of a story as this may have been, there are certain facets of the environment (and inconsistencies on the plot) which don't add up. By recognizing the arc's flaws, we can appreciate its downsides along with what made it engaging and memorable.
10 Yellow Sky
One of the most peculiar though nonetheless palpable features of part four was that the entire sky was yellow. This was an exceptionally unusual artistic choice as it is not shared by its predecessor, Stardust Crusaders, nor its successor, Golden Wind.
An argument is often made that this is to illustrate the looming sense of "wrongness" over the town of Morioh, though given that its disappearances are solely due to the machinations of Yoshikage Kira, this still does not explain why the sky has been influenced.
9 The Stand Arrow Never Killed Anyone
As stated by Okuyasu's older brother, the Stand Arrow is more likely to kill someone than awaken the power within them. However, when it was actually applied in the anime (whether by Keicho Nijimura or Yoshihiro Kira), the converse was true.
An especially egregious example comes from when Keicho shot Koichi through the neck with the Stand Arrow. While his target may have had the perseverance to awaken his power, he still had a physical trauma wound which should have killed him in the several minutes Josuke needed to save his life.
8 Heaven's Door
Heaven's Door is one of the most outrageously powerful Stands in the entire JoJo universe. It can rewrite any facet of one's condition through inscribing on them, as well as glean a synopsis of their entire lives.
However, Rohan did not use it to its fullest potential. He could have greatly enhanced the strength of his allies on a whim, including allowing Josuke to use his Stand's healing properties on himself or by making Okuyasu smarter (and therefore dramatically improving how deadly his Stand can become). Unfortunately, it would never be utilized for these purposes.
7 Yoshihiro Kira
When analyzing the character of Yoshiro Kira, there are fewer things that do make sense than don't. Though its somewhat believable that he could be a ghost haunting the spiritual world (even if Reimi Sugimoto would curiously not have the same luxury of mobility as he does), it becomes more perplexing still when analyzing his usage of the Stand Arrow.
Not only is he able to physically hold it as an apparition, the arrow itself becomes smaller in his grasp. Further, he will often commentate on the battles between his minions and Josuke's allies despite not being a Stand user himself. Therefore, he should not be able to see the Stands of those who are fighting.
6 Yuya Fungami's Survival
Yuya Fungami's Stand, Highway Star, was able to drain the nutrients of those it touched and almost restored him to perfect health. After he defeated Rohan Kishibe, Josuke raced to his hospital to confront him in person and finish the fight.
Though Josuke healed him before the inevitable beatdown (as he deemed it sporting), he should not have survived the fall which ensued. Fungami plummeted from one of the highest floors of a towering building with only a thin, conveniently placed fountain to break his fall. Realistically, it should have killed him immediately.
5 Yoshikage Kira's Enhancement
The function of the Stand Arrow is to unlock the Stand of those who don't currently have one, not to give a new ability to those who do. During Kira's moments of greatest despair immediately after he was forced to murder his "adoptive" son, Hayato Kawajiri, his father approached him and accidentally stabbed him with it. The shaft slid up his body and imbued him with the "Bites The Dust" ability.
Not only was it never explained how the Arrow could penetrate Yoshikage's arm in such an unusual way, it defied the established rules on how it was supposed to work.
4 Okuyasu's Father
Okuyasu's father was cursed with an immortal life of deformity. No matter what happened to him, he would always recover, returning to his repulsively addled state .
However, instead of attempting to cure their father, Nijimura was more interested in finding a Stand that could destroy him (as he had given up hope on his restoration). This was an incredibly obtuse quest, since if even Okyasu's Stand could not put him out of his misery (and Josuke's couldn't heal him), it's unlikely that anyone else would have better luck.
3 Josuke's Game With Rohan
Josuke challenged Rohan to a gambling game under the premise that his "dice" were rigged. He used the ability of a new friend he made, Hazekura, to rig his own victory.
Kishibe suspected he was cheating after an incredible stokes of "luck", determined to uncover his secret even at the expense of his own house. If he had used his Stand, Heaven's Door, he could have simply read the answer from Josuke's face and uncovered the truth behind his cheating.
2 Koichi's Stand Progression
Koichi's "Reverb" Stand is one of the strangest in the series. In addition to "evolving" naturally (unlike Yoshikage Kira's, which was due to the power of the Arrow), the first two transformations are starkly contrasted to the last.
Reverb Act I allows him to play a sound repeatedly, Act II lets him write words on a surface (with those who touch them experiencing the definition of the text, such as feeling a burn by touching the word 'heat')< and Act III enables him to crush his opponents by enhancing their gravity. The final evolution of his Stand is nothing like its previous two iterations.
1 Yoshikage Kira's New Life
Yoshikage Kira was able to escape the clutches of Star Platinum because he forced Cinderella to swap out his face with that of a nearby bystander's. Though it's suspiciously convenient (though still believable) that one would be nearby with a similar height and build to exchange himself with, it does not account for how he could embrace his new life easily.
There is too much information for him to have to know for a seamless transition to be possible. Examples include the names of his coworkers, the skills required to do his job correctly, and even the years of marital experience he could have no idea how to feign. Further, the Cinderella Stand did not change his voice, which should have been a dead giveaway to his real identity.