For some of the less discussed books, length has been an issue. I personally try to nominate books that are <350 pages. Something that most people (even the non-bookworms or those that don't have hours to read every day) can read in a week or two without much problem.
For Harry Potter the number that have already read it is probably high. I don't really want to re-read it because I will be doing that when my son gets older.