1/ Avoid depressing books (not in the sense that they have a pessimistic, misanthropic, depressing tone/vision, but in the sense that they deal with painful, tragic subjects):
a) The majority of the greatest, most critically acclaimed, most valuable books, are depressing. One who has this attitude thus misses a great number of extraordinary works.
b) One who avoids depressing books often says that it's unnecessary to read about cruelty, misery, sadness, agony... when they have seen such things in real life. That is wrong. You may know pain and suffering but in life there are lots of other kinds of pain and suffering. Through reading one learns to empathise with people, to see things from other people's points of view, to widen one's perspective.
c) This attitude is equivalent to the choice to be kept ignorant, to be blind to suffering and all the negative aspects of life, all the negative things happening around the world.
d) Avoiding depressing books also means a 'disapproval' of, denial of and fear for feelings labelled as negative and thus leads to a pointless pursuit of so-called happiness, which is more like a kind of happy-go-lucky, light-hearted attitude, instead of pursuit of wholeness.
2/ Always want a happy ending (more extreme: avoid reading books that don't have happy endings).
Regarding the readers, they'll be like the people in no.1 above.
Regarding the writers, some don't know that a forced happy ending may ruin the whole work.
Eg: "Twilight" saga (Stephenie Meyer), etc.
3/ Wish to like and identify with the characters, at least the protagonists (and thus dislike those in which one doesn't like any characters or doesn't like the protagonists).
This is very common, and I myself don't understand why to some people novels can't be appreciated objectively for their merits, their values, their significance, but have to be perceived personally, and thus these people may attack a book and treat it as annoying or worthless simply because they don't like the people in it.
There are times when I love a book even though I don't like anyone in it, such as "The great Gatsby" (F. Scott Fitzgerald), "Wuthering heights" (Emily Bronte), and even if I strongly dislike the main characters/ protagonists, such as "Lolita" (Vladimir Nabokov), "The piano teacher" (Elfriede Jelinek), "Madame Bovary" (Gustave Flaubert), "The stranger" (Albert Camus)... Or concerning the case of "Anna Karenina", I sympathise with Anna and don't want to blame her because I understand her position and emotions, but I don't always like her, Anna has her faults and shortcomings that somehow also increase her afflictions and contribute to her downfall. But that doesn't matter. The question of whether or not I like the characters, in my opinion, doesn't have much to do with the appreciation of the work itself. Elfriede Jelinek and Vladimir Nabokov may each create and depict a character nobody can like, Erika Kohut and Humbert Humbert respectively, but these 2 talented writers should nevertheless be praised for their understanding of psychology, for their exploration of the character's psyche and personality.
But well, that's my rant.
To each his own, I'm afraid. I can't change anybody.