It’s a widely recognized world heritage site. A whole country took its name. It’s a world Wonder in the game Civilization.
Yet when I looked into it, it seems really unremarkable. By the time it was founded the Arab world was building delicate mosques, China had huge palaces, and Europe was perfecting its castles.
By comparison, the great Zimbabwe looks like a (very) small fortified city with a large granary.
It seems to me that we are judging it on a curve. No doubt it was remarkable for its inhabitants to construct it but although it’s regionally noteworthy, I’m not yet convinced that it stands on a global scale.
So what’s the deal? Why is it significant?
So, the "great" in Great Zimbabwe comes about because there are about 200 stone structures ("zimbabwes") scattered across the zimbabwe plateau and limpopo valley, and Great Zimbabwe is the largest/most extensive of these structures. Also it was the capital of the Zimbabwe kingdom from the 12th to 15th centuries.
By comparison, the great Zimbabwe looks like a (very) small fortified city with a large granary.
The size of the city is quite a bit larger than you imagine.
The stone buildings of the Great Enclosure, the Hill Complex and the Valley Complex make up a footprint of about 1 square kilometer. However, at it's height, the total settlement footprint of Great Zimbabwe is approximately 25 square kilometers^1 and supported a population of upwards of 18,000 people.^2 Needless to say, this larger settlement area was made up of perishable housing materials that aren't as immediately visible as the stone structures. The stone enclosures thus represent a "core" or acropolis, and the Great Enclosure is better understood as a palace wall to denote the power and separateness of the monarch from the rest of the population.
So what’s the deal? Why is it significant?
You have mentioned that it was inscribed as a UNESCO World Heritage Site. On the UNESCO website, the site webpage includes the criteria why it was included on the list. Briefly they are:
it represents an outstanding example of the drywall mortar-less construction technique that was developed as an independent architectural tradition. There are some 200 of these sites, some of which are several centuries older, some were constructed in the 1500s and 1600s after Great Zimbabwe was abandoned. However, Great Zimbabwe is included as the largest and most impressive example of this building tradition.
The heritage list takes special effort to make note of the works of world civilizations. Great Zimbabwe is included to represent the "lost"* civilization of the Shona people.
It is a site that is extremely historically significant to the population of Zimbabwe, a site and culture that they identify with.
Edit:
I should add, a large part of the notoriety about Great Zimbabwe comes from historical and archaeological debates about its origins.
In the very first written account we have about Great Zimbabwe, by Portuguese explorer Joao do Santos in 1609, he links the site to King Solomon's mines. It makes a little bit of sense if you consider the limited perspective of the time period, that he would try to connect a site in a region known for producing gold to a biblical story.
However, there was a pattern in the 19th and early 20th centuries of attributing the construction of Great Zimbabwe to various middle eastern or "hamitic" populations. At the time, the vast majority of anthropologists and historians were unable to accept the idea that Africans had the ability to build such monumental architecture, given that local Shona and Ndebele populations weren't practicing monumental stone construction in the 1890s when Europeans arrive in the region. (I say most, because there were archaeologists like David Randall-McIver in 1905 and Gertrude Caton-Thompson in 1931 who declared Great Zimbabwe's African origins.)
So, for much of the 20th century Great Zimbabwe was treated as a mystery, and the riddle was to figure out what culture was responsible for its construction and when it was built. After the Unilateral Declaration of Independence, Rhodesia's white government became very invested in this research, and attempted to suppress or discredit any research that pointed to African origins of the site.^3
-edit
1 Urbanism in the Preindustrial World: Cross-cultural Approaches. pp 154.
2 The Zimbabwe Culture by Innocent Pikirayi. pp 64.
3 Great Zimbabwe by Martin Hall and Rebecca Stefoff. Oxford University Press, 2006. PP 39, 41-42
*of course, talking about it as a "lost civilization" can be problematic. The current academic consensus is that the builders of Great Zimbabwe were the descendants ancestors of the modern Shona population of Zimbabwe (perhaps also closely related to the Venda of South Africa too). Shona lifestyles and settlement patterns have obviously changed quite a bit due to colonialism and independence, but they are still there.
edit- changed "drywall construction" to read "mortar-less construction"