At any rate, and laugh lines aside, a number of aspects make reading books challenging.
Long sentences. Run along sentences. Absent punctuation, thus no sentences. Or too much punctuation, thus altering the sentence altogether. Generational jargon, such IMHO and BTW, which by-the-way it took the longest to figure that one out. Generational gaps is another one; i.e., Gen X...the millennials...the baby boomers...and that silent gen. International differences can make reading challenging, along with cultural incongruences... Artsy Writing, or Creative Writing... Scenic Writing. Padding. Data Dumps. Clichés. Big Words. Offensive Words. Words with a half dozen meanings. Big books. Heavy Books. Pages upon pages of ad hominems. Fallacies Galore, No humor, whatsoever. All of these conditions, and more for sure, contribute to various forms of reader anxiety.
Now, I just finished reading a book that on sight, in all truth and honesty, intimidated me...although for one reason, and one reason only. ‘Long Walk to Freedom’ by Nelson Mandela (my thoughts here) sat on my to-read shelf for at least a year because that book looked, and felt, like it weighed a ton! Surprisingly, and much to my delight, the book was heavy, but not challenging.