From the World Heritage inscription:
Joya de Cerén is remarkable by virtue of the completeness of the evidence that it provides of everyday life in a Mesoamerican farming community of the 6th century AD, which is without parallel in this cultural region. It was a pre-Hispanic farming community that, like Pompeii and Herculaneum in Italy, was buried under a volcanic eruption about AD 590. Although a warning earthquake apparently gave residents time to flee, the ash preserved their personal belongings, from garden tools and bean-filled pots to sleeping mats and religious items, essentially freezing the agricultural village in time. Because of the exceptional condition of the remains, they provide an insight into the daily lives of the Central American peoples who worked the land at that time.
I have visited several archeological/paleontological world heritage sites around the world. While most of them are of great scientific import, they usually make for very poor tourist attractions. The reason why the locations are important is because of artifacts long since dug out of the ground and put in museums. You almost never are able to see one of the actual dig sites and the best you can hope for is to visit a museum. I have visted two sites like this previously: the Sangiran Early Man site in Indonesia and the Ban Chiang site in Thailand.
Joya de Ceren is superior to those sites in that you can seen the actually buildings which have been excavated.
While it is often called the Pompeii of the Americas, don’t visit Joya de Ceren expecting to have the same experience you would see at Pompeii. The site is much smaller (although an estimated 90% hasn’t been excavated yet) and the buildings are much less impressive. Also, all the structures are kept under an awning to protect the structures from the elements. This makes for less than stellar photography.
The site can easily be visited as a day trip from San Salvador. It is about an hour drive from the city center.
View the complete list of UNESCO World Heritage sites I’ve visited.