What makes a book unreadable?

I'm currently reading "the Brief Wondrous Life of Oscar Wao" by Junot Diaz, and this question came to mind.  The book, a Pulitzer prize winner, is about a Sci-Fi/fantasy geek; a fat, Dominican, ghetto-nerd and his love-seeking adventures.  I picked up the book after seeing it many times, and was really looking forward to enjoying the story... And so far I'm not really into it. Why? Because of the language.  I'm not a linguistic prude by any means; I curse like a sailor on a regular basis.  But, I am having a hard time getting past the words. The book is in third person, not narrated by Oscar himself, and I believe there is a big disconnect between how I feel he would speak and how the narrator talks.  And it just disturbs me! A ghetto-nerd sounds so innocent, and his language is downright filthy.  Needless to say, I have not really gotten that far in the book, which is disappointing, because the story is good.  In the end, I think I'll plod through, for the story's sake, but I got to thinking, "What makes a book unreadable?"  Here are the reasons I have stopped reading a book:

1. Writing Style

A lot of time when I read, the author's writing style affects my mind.  I mean, I read their writing and I begin to think in that way.  If the writing style really doesn't jive with my thought processes, I can't continue reading.  I especially cannot stand authors who seem to be using big words just for the sake of using big words.  You don't need to show me that you can use a thesaurus.

2. Redundancy 

I cannot stand hearing authors repeat themselves over and over again.  I remember what you said the first time!  I almost dropped "The Sugar Queen" for this reason. I think that being repetitive shows a lack of writing skill as well as poor editing. Inexcusable. 

3. Disgusting Content

Again, I am not a prude.  I can read about revolting things, but I have to draw the line sometimes.  For example, I can read about killing cats, like in a certain Murakami book, but I couldn't finish the third story in the novella "Raise the Red Lantern" after the author started discussing the collection of bodily fluids in jars.  Ewwww.

4. Weak Main Character

If I don't like the main character, I don't like the book.  It just doesn't work. I hate when a main character is whiny and annoying through 2/3 of a book and then suddenly becomes strong.  Try again, author, try again. Example: Bella Swan from Twilight. Oh, Stephenie Meyer, you are so lucky we like Edward, or your books would not have been read.

There are probably many other reasons I end up not finishing books, but I feel that these are the most obvious examples I have found so far. I think that reflecting on this is good for me as a reader but also as a writer.  I'll keep it in mind. :)

What makes a book unreadable to you, dear reader?