Answer:
The main difference is in style. Traditional literature has long, flowing, sentences and long paragraphs. The descriptions are more in depth, often painfully so, and often what would be considered purple prose. The pacing is usually slower. Subject matter may have been more limited.
21st century literature is more compact both in structure and style. Sentence structure is often short, the pacing is faster in moving the story along. (I, personally dislike strings of short sentences. It reminds me of reading a greeting card.) Subject matter has more latitude in dealing with things that would have been considered taboo in traditional works.