These are what we call the 'YMMV items.' Things that some people find in this work. We call them 'your mileage might vary' because not everyone sees these things in the same way. This starts discussions in the trope lists, a thing we don't want. Please use the discussion page if you'd like to discuss any of these items.
Ensemble Darkhorse: Lampshaded. Uekasa believes he's one, but he's probably the least popular of the Ryonan players.
Fanon: No, it's never stated that Mitsui has to repeat his senior year at any point in the manga. Fans commonly misinterpret the last chapter (Akagi and Kogure quit the team to study for exams while Mitsui stays and feels "lonesome") as meaning Mitsui having to stay at Shohoku for another year. In 10 Days Later it's revealed he's staying for the Winter Tournament (which takes place during the second semester of the school year) so that he he can possibly get a scholarship to a university instead. Although it's possible Mitsui later repeats senior year, it's not canon.
In one of the earlier chapters of Slam Dunk, Miyagi beats a delinquent half to death for slapping Ayako, declaring that you never hit a girl. When he was eleven he slapped her for littering.
At the end of the one-shot, Ayako tells him to keep a picture of her on his desk so he can see her everyday; he thinks this is an absurd idea. Five years later, he not only keeps a picture of her on his desk but talks to it every morning.
They Wasted a Perfectly Good Plot: Just before the nationals story arc, the various teams that will be playing are introduced, featuring the top players and even offering a much stronger rival for Sakuragi. And so it was a shock to many fans that it turned out that Shohuku lasts for only three games, with the second one being the Grand Finale and the third (where Shohoku lost) being only mentioned right before the epilogue.
The Woobie: Yasuda's shy and good-natured which naturally means he's pushed around in a team full of alpha males.
Uozumi (his backstory oif loneliness and school bullying is a Tear Jerker)
God, does Youko need a hug when you see her flashbacks in the first movie. Thank God Oda straightened up his act.