Many features–including topic and author’s style. But the variable that consistently predicts reading comprehension is vocabulary. Remember that the core vocabulary accounts for at least 90% of the words in most texts.
The other words in texts come from a group of around 300,000 words—words that are rare in written English. An additional one or two rare words per 100 words can add challenge. The kinds of words, of course, make a difference, especially when most of the rare words are multisyllabic rather than mono-syllablic
Further, students are rarely asked to read texts that are only 100 words long. The number of rare words needs to be viewed relative to the entire task.
The rate of rare vocabulary matters in text complexity and rare vocabulary can be taught as illustrated in numerous resources at www.textproject.org.