I agree with what has been said. The most important is the development of the plot along-with the characters. I have found that some authors get into the descriptions so boringly that you don't feel like reading. Others do a fabulous job making the descriptions also a part of the narrative. For me if a book can't hold my interest in the first 20-25 pages, it never will. I have learnt the hard way not to hope for things to improve later in the tale! It usually disappoints.
Even non-fiction can be made quite interesting e.g. Innumeracy by John Allen Paulos.
I would say it's the plot of the book and also how the author makes sure that he doesn't divert from the actual plot and also how the author keeps us engaged in reading the book makes it a good book.