The reason the Mona Lisa is so famous is because Vasari waxes serious lyrical about it in his Lives of the Artists (published in 1550). Here's what he says about it:
"Anyone wishing to see the degree to which art could imitate nature could readily perceive this from the head; since therein are counterfeited all those minutenesses that with subtlety are able to be painted: seeing that the eyes had that lustre and moistness which are always seen in the living creature, and around them were the lashes and all those rosy and pearly tints that demand the greatest delicacy of execution. The eyebrows, through his having shown the manner in which the hairs spring from the flesh, here more close and here more scanty, and curve according to the pores of the flesh, could not be more natural. The nose, with its beautiful nostrils, rosy and tender, appeared to be alive. The mouth with its opening, and with its ends united by the red of the lips to the flesh-tints of the face, seemed, in truth, to be not colours but flesh. In the pit of the throat, if one gazed upon it intently, could be seen the beating of the pulse: and indeed it may be said that it was painted in such a manner as to make every brave artificer, be he who he may, tremble and lose courage. He employed also this device: Mona Lisa being very beautiful, while he was painting her portrait, he retained those who played or sang, and continually jested, who would make her to remain merry, in order to take away that melancholy which painters are often wont to give to their portraits. And in this work of Leonardo there was a smile so pleasing, that it was a thing more divine than human to behold, and it was held to be something marvelous, in that it was not other than alive."
> The eyebrows, through his having shown the manner in which the hairs spring from the flesh, here more close and here more scanty, and curve according to the pores of the flesh, could not be more natural.
This is quite interesting, because (currently at least) she doesn't have eyebrows!
The Louvre’s Mona Lisa is in shitty condition and poorly preserved. The Prado’s Mona Lisa is thought to be the earliest replica of the original, and has been masterfully restored. There’s no question the La Gioconda has eyebrows (that match the above description) there.
While the background of Prado’s copy was blacked out in the 1700s prior to its restoration this past decade, the foreground was largely untouched.
I share that opinion and wholeheartedly agree, but I refrained for fear of controversy derailing the point. If it were "merely" a very early replica that was infinitely better preserved, it would still be of better historical record value.
"Anyone wishing to see the degree to which art could imitate nature could readily perceive this from the head; since therein are counterfeited all those minutenesses that with subtlety are able to be painted: seeing that the eyes had that lustre and moistness which are always seen in the living creature, and around them were the lashes and all those rosy and pearly tints that demand the greatest delicacy of execution. The eyebrows, through his having shown the manner in which the hairs spring from the flesh, here more close and here more scanty, and curve according to the pores of the flesh, could not be more natural. The nose, with its beautiful nostrils, rosy and tender, appeared to be alive. The mouth with its opening, and with its ends united by the red of the lips to the flesh-tints of the face, seemed, in truth, to be not colours but flesh. In the pit of the throat, if one gazed upon it intently, could be seen the beating of the pulse: and indeed it may be said that it was painted in such a manner as to make every brave artificer, be he who he may, tremble and lose courage. He employed also this device: Mona Lisa being very beautiful, while he was painting her portrait, he retained those who played or sang, and continually jested, who would make her to remain merry, in order to take away that melancholy which painters are often wont to give to their portraits. And in this work of Leonardo there was a smile so pleasing, that it was a thing more divine than human to behold, and it was held to be something marvelous, in that it was not other than alive."