During its time the Mona Lisa was a first of its kind. Women depicted in portraits prior to the Mona Lisa did not look directly out at the viewer. During the Renaissance it was considered wrong for a woman to talk to a man other than her brother, father, or husband. This belief was reflected in the way portraits of women were done by not having them look out at the viewer, they didn't want these women's eyes used as an invitation to men, who were the ones that bought and mainly looked at artworks. Leonardo was known for breaking boundaries and inventing new ways of doing things. He decided to paint Mona Lisa looking right at the viewer, with that infamous coy smile. After he made that painting it was so popular that people started changing the way they wanted portraits of women done.
Also, during that time the Mona Lisa was actually considered very attractive. The image of ideal beauty from that period was of a woman with a high forehead, pail skin, and blond hair. The women would often shave their eyebrows and hairline to extend their forehead. Shaved eyebrows are far from attractive today. Also, the Mona Lisa does not have the ideal blond hair or pail skin that can be found in portraits of women before this painting. This was another boundary Leonardo pushed, changing the ideal beauty from fair skinned and light hair, thus showing that a woman could be attractive with darker hair and skin as well.
So to answer your question, the reason why it was and still is so valuable today is because not only is it made by Leonardo, one of the greatest artists and inventors of all time, but also this painting broke so many rules, and is in fact a wonderful painting. There is much more to it than what I just described, for example there is also a lot of aesthetic value and innovative techniques created by Leonardo in this image, like his sfumato effect, but I'm not going to go into all of that. Just wanted to give you an (relatively short) answer.