I would just add two other points to those already made.
1 - Da Vinci was known for lots of little painterly tricks and hidden messages, hence the ease with which his work was used for The Da Vinci Code.Some believe that the Mona Lisa may be a self portrait(in the female) of Da Vinci himself as when a famous self portrait of his is overlaid, the features are notably similar.
2 -The Mona Lisa was one painting that Da Vinci never attempted to sell. He took is almost everywhere with him and was indeed with him when he died, which is how it ended up in France. A peice of work that the artist himself rates higher than all of his other works naturally adds interest to the artwork.