There are many special things about the Mona Lisa painting. The foremost one, in my opinion, isn't necessarily the painting but the person who painted it: Leonardo da Vinci.
He is known to be one of the world's greatest polymaths ever who, among various skills, could paint and draw exceptionally.
(Courtesy: The Independent)
Although his two paintings -- Mona Lisa and The Last Supper -- usually steal all the headlines, the work of Leonardo da Vinci includes a lot more, including Leda and the Swan, St Jerome in the Desert, The Virgin of the Rocks, and Portrait of Ginevra de' Benci. Aside from paintings, he was great in science, geology, astronomy, engineering, sculpting and more.
So, a lot of attention that Mona Lisa painting attracts isn’t for its marvel itself but rather a genius who brought it into reality. Leonardo da Vinci, even centuries later, remains a fascinating figure among researchers across different fields. He still stands as an individual, with great historic significance, who a lot isn’t known about; there still remain many mysteries around the name.
So, Mona Lisa painting is a point of fascination and special attention for many because of Leonardo da Vinci.
But that isn’t to say that the painting itself isn’t a piece of magnificence. One of the most special things about it is her eyes and optical illusion in them. Painted using a special technique called 'silberblick', the eyes follow you wherever you move.
Another special thing about Mona Lisa painting is the rare technique Leonardo da Vinci used to make it. It was the primary reason why it took him 4 years to paint her. Called 'sfumato', there are only a handful of people on earth who has been able to master this painting technique.
(Courtesy: National Geography)
On top of these, countless events over the years and centuries have helped build quite an interest around the painting. For instance, in 1911, it was stolen for 2 years. It made the headlines across the world. (At the time, Pablo Picasso, a legendary painter, was (wrongly) under suspicion for the theft.) Also, in 2015, another Mona Lisa painting was discovered in St. Petersburg, which gave birth to many conspiracy theories.
So, while the painting itself is a masterpiece that has inspired thousands of artworks across the world, there is no one reason why it is so special. There are many reasons -- including different individuals, incidences, and circumstances.
Hope this answers your question.