This is simply based on my personal opinions so take them with a grain of salt. I am not here to diss anyone for having these odd characteristics, but you do you!
1) Buying books for display (and never reading them).
Are they just props you could put in your room and say "Tee-hee! I like books!" to your audience?
Maybe you like to collect books but now they're already gathering dust since you haven't opened them in a while. You only bought them because of the shiny covers.
Now, you wanna buy more ... like why?! It's a waste of space.
2) Folding pages.
Just no! Please, for the love of God, don't do this! The publisher and the author both worked hard to bring you the finest books but only for a kid to fold the pages like origami. Use a bookmark or a colorful sticker to mark the page.
3) Doodling or scribbling on pages.
I remember picking up a book in middle school and finding random pencil drawings or highlighter markings on the pages. I get you might be bored and simply doing this to note what you're reading, but this isn't a sketchpad! It makes the book look cheap.
Thank God we have ebooks like in Kindle and Kobo which allow you to scribble away.
4) Hate-reading.
If nobody's forcing you, then what's the point of picking up a book you don't like? Instead of giving constructive criticism, you resort to insulting the author and their work.
There are also instances where many people don't bother giving the writer a fair shake as they already assume the book is bad. Why not write your own book instead?
5) Treating fiction as reality.
You are the kind of reader who hates on a character and complains about how "unrealistic" the story is. 'Fiction' is the opposite of reality.
There are a few exceptions to this rule, such as how a story is supposed to be an autobiography, historical (e.g. the experiences of Korean War veterans) or a slice-of-life drama meant to accurately portray high school in the 1980's.
6) Headcanons.
While there is nothing wrong in imagining a character as gay, Asian, autistic or a mere psychopath, at the same time I can't help but wonder what makes them think someone is such based on a few superficial traits. Unless the creator/s themselves confirmed it as canon.
Oh, Wednesday Addams has trouble making friends and acts weird? Autistic. Elsa doesn't have a boyfriend? Lesbian. Scott Pilgrim is loud and hyper? ADHD. Sure, that might be the case but to me, these characters could just be weird, silly or aromantic.