I had about three main issues with this book:
The first issue I had with the book was that it had no main plot. The book plods and drags along with self contained min-arcs with no common conflict linking them together. And no, it's not about the rape trial. It doesn't even begin until the rear end of the novel, and only one interesting thing happens before the trial that's related to it: the scene where Scout shames an angry mob away. I found it stupid; what kind of proper group of murder-minded grown men with guns would let themselves be driven away by a child? The only constant throughout the novel is the characters' interactions with Boo Radley, which even then are indirect and sporadic.
The second issue is the lack of emotional depth. The mini-arcs I complained about in the previous paragraph are quite boring, and even the rape trial is highly bland. The reason is that it's extremely obvious Tom Robinson is going to be convicted due to a racist jury. And while he's innocent, we never really get to know him as a reader, which makes it hard to care. The attempted murder of two of the main characters later on brings out no emotion, as anyone who remembers the opening sentences knows Jem survives, and Scout (she's a child, a girl, a "good guy", one of the main characters, and she's the narrator speaking in past tense) has Plot Armor
thick enough to deflect the Death Star's superlaser.
The third issue I had was that the themes were common sense or things people learn just by living. Racism is evil? There's no reason to believe otherwise. People are more than what they seem? Of course not everyone shows all they are on the surface. The trials of growing up? That's something (most) people do on their own and experience firsthand when they do it themselves and experience it secondhand with many real people (if they're around children a lot); what's the point of watching a fictional person do it? Overall, the themes just didn't do it for me.
I'm not complaining just to troll, and I respect the many people who love this book, it's just that I can't find anything good about it.