Of the hundreds of buildings at the Shanghai Expo, the British pavilion, designed by Thomas Heatherwick and dubbed the “Seed Cathedral,” has become the darling of the design world. Not only is the building—studded with 60,000 acrylic tubes—visually compelling, but the tubes contain seeds (from Kew Gardens’ Millennium Seed Bank Project) that will be dispersed to Chinese schoolchildren after the Expo closes, making it an elegant comment on biodiversity. Critic Brooke Hodge, writing in The New York Times, called it the Expo’s “only national pavilion that truly integrates concept, container and content.”

It is also a perfect example of the architectural experimentation that world’s fairs have long made possible. Who knows whether the thousands of tubes, inserted into a plywood box, would last through even the mildest Shanghai winter? And who cares, given that virtually all of the Expo’s buildings, on a two-square-mile site flanking the Huangpu River, will be torn down by the time winter comes around. Heather­wick’s pavilion demonstrates what has, for more than a century, made World’s Fair architecture so intoxicating: temporariness.

Yes, that means there is something terribly extravagant about expos; in this case, more than 200 buildings will close forever on Oct. 31. At the same time, the temporariness is liberating for architects, who get to experiment with forms and materials that aren’t required to stand the test of time—though sometimes, triumphantly, they do. The Eiffel Tower, gateway to the 1889 Paris expo, was meant to be temporary; the requirement that it be easy to dismantle explains its distinctive Erector Set appearance.

But that’s not the only form of freedom afforded Expo architects. There’s also a complete absence of NIMBY-ism—not just because this is China (where almost nothing is allowed to stand in the way of progress), but because Shanghai is already a kind of architectural theme park. Buildings are outlined in flashing LEDs and bridges are lit like amusement park attractions; skyscrapers struggle to outdo each other not just in height but in outlandishness. Restraint was hardly expected of the architects invited to this party.

Then, too, the theme of the fair is “Better City, Better Life”—a phrase that resonates deeply in a society undergoing the most rapid urbanization in history. One focus inside the German pavilion is Herzog & de Meuron’s concert hall in the HafenCity area of Hamburg; in the vast “urban best practices” zone, representing 55 world cities, a Bilbao-sponsored tribute to Frank Gehry is one highlight. And the massive Expo-sponsored themed pavilions offer a complete education in city planning. It would take a month to see not just the architecture, but the architecture-related exhibitions, at this expo.

Of course, the Expo is about far more than design—it is a brilliant act of international diplomacy, designed to show visitors (most of them Chinese) that they have friends around the world. And, thanks to subsidies from the organizers, even the world’s poorest nations are strutting their stuff. Enter what is called the Africa Joint Pavilion–provided by the Expo to dozens of African countries at a reported cost of $100 million—and you can feel the pride of nations like Chad, Malawi, and Djibouti, which have rarely had such prominence on a world stage. Long after the Expo has closed and its site turned into a high-end residential neighborhood, China will be reaping the rewards of its outreach to poor (but in many cases resource-rich) countries.

Yet with nearly 60 major buildings (and with a daily population of up to 600,000), the Expo is a city in itself, not just an exhibition about cities. And so it’s fair to ask what kind of example it sets. Here’s a scorecard:

Infrastructure: Signage is superb, almost completely eliminating the sense of disorientation a large city can induce in first-time visitors. Maps at three scales—imitating the zoom effect of Google Earth—are everywhere. And other amenities, from bathrooms to wheelchair rental sites to fountains dispensing chilled, purified water, are plentiful.

Transportation: Ferries running between the two sections of the fair—the larger area on the Pudong side of the river, the smaller on the Puxi side—are streamlined and efficient. On land, however, the only options besides walking are electric buses, which run in loops far from many of the pavilions, and oversized golf carts available to VIPs. The absence of a monorail or tram is surprising, especially given the kinds of transportation available elsewhere in Shanghai, where a Maglev ride from downtown to the airport takes just seven minutes.

Safety: With the eyes of the world on China—where poor construction led to thousands of deaths in Szechuan Province after the 2008 earthquake, and (less tragically but almost as visibly) a fireworks display destroyed part of the CCTV showpiece by Rem Koolhaas—the organizers have been obsessive about safety. Architects reported countless meetings with code enforcement officials to get the approvals they needed.

Open space: The density of the Expo is balanced by a huge new park stretching more than a mile along the Huangpu River. Called the Houtan Garden, it is a mix of natural and man-made elements reminiscent of other great new urban parks (including New York’s High Line). A bookend to a handsome new promenade along the city’s Bund, the garden will remain in place after the expo closes.

Originality: The Expo has been plagued by charges of plagiarism. The mascot bears a striking similarity to Gumby; the theme song was based too closely on a Japanese pop tune. And the master plan bears strong resemblance to one submitted by Frederic Schwartz, the New York–based architect, in a competition sponsored by the organizers in 2004. Schwartz said that he was paid $50,000 for his work, but never received another cent, much less a contract, after the Expo organizers ended the competition without naming a winner. With controversies like this, the fair did nothing to assuage architects wary of working in a country with a spotty record of respecting intellectual property rights.

Sustainability and historic reuse: It’s hard to apply the term “green” to an event expected to draw 70 million visitors to hundreds of temporary buildings. (The 2005 expo, in Aichi, Japan, consisted almost entirely of prefab buildings that could be recycled after the fair, a more direct nod to sustainability.) On the other hand, urbanization—the selling of which is a key agenda of the fair—is greener than suburbanization, a scary alternative in a nation of 1 billion people.

Perhaps the biggest surprise of the Expo is how many existing buildings were repurposed, part of a conscious effort by the organizers to demonstrate environmental stewardship. On the Pudong side, a former factory building was given sleek new appointments as a joint pavilion for 10 Latin American countries (including Panama, with a working canal model). Also on the Pudong side, a former steel mill became an impressive indoor-outdoor sculpture garden, while old gantries morphed into Calder-esque constructions.

On the Puxi side (the smaller of the two fairgrounds), the examples of reuse are even more spectacular. A monumental shipyard building, its steel beams as thick as railroad cars, became the China State Shipbuilding Corp. pavilion, filled with high-tech displays on the history of ocean voyaging. A few hundred yards away, a factory building with a towering smokestack became the Pavilion of the Future, with a central space that resembles the turbine hall of London’s Tate Modern museum.

And what of the national pavilions? A few countries (Vietnam, Pakistan, and Thailand) produced buildings in “historical” styles that seem out of step with the rest of the Expo, with its relentless focus on the future. Oddly, China itself was tripped up by an attempt to use historical motifs. The 200-foot-tall China pavilion—the so-called “Oriental Crown,” designed by He Jingtang—is a monumental extrapolation of traditional dougong, or wooden brackets. Up close, its giant cantilevers (56 of them, representing China’s 56 recognized ethnic groups) feel menacing; from a distance, especially at night, when bare strip windows dominate the façades, it resembles an office building (and in a city where office buildings look like world’s fair pavilions). After the Expo closes, this pavilion will become a museum, making it one of the few structures to escape demolition.

At the other extreme from the hugely extravagant Chinese pavilion are the prefab buildings occupied by countries with modest budgets. Some—including the pavilions of Croatia, Cambodia, and Belarus—are sheathed in billboard-sized photos. In other cases, the ugly ducklings are transformed into swans with ingenious façade systems, including Serbia’s, which employs snap-together modules reminiscent of Legos.

The U.S. pavilion, which looks like a suburban auto dealership, is just one step up from the prefabs. The pavilion is the result of a 1999 law that makes it hard for the U.S. government to spend money on international expos, a short-sighted bit of isolationism that has led to a series of embarrassments. A nonprofit corporation struggled to raise money to build it, and—with funding unsure until the 11th hour—commissioned a no-frills building by the Canadian architect Clive Grout, whose firm specializes in pavilion and “attraction” design.

The building Grout produced isn’t the complete humiliation some predicted: It has a graceful shape and a high-tech sheen. But it’s hardly a beacon of U.S. ingenuity. By contrast, there are some strong showings from south of the border. Chile put up an urbane pavilion (by Sabbagh Arquitectos) of recycled wood and Cor-Ten steel that makes you want to hop a plane to Santiago. Mexico’s witty pavilion (by the firm Slot) is a series of colorful, kitelike umbrellas, shading a slanted lawn (with galleries below).

Two pavilions that get visitors moving are big draws: Denmark’s pavilion, by Bjarke Ingels of BIG, is a giant spiral velodrome (picture an open-air Guggenheim Museum equipped with hundreds of bicycles). At the center of the vortex is the Little Mermaid, the Danish icon, shipped in from Copenhagen—the statue’s first voyage from home. The Swiss pavilion, by Buchner Bründler, was designed to accommodate a ski lift, which makes a 10-minute circuit through the building to a rooftop Alpine meadow.

But even large expenditures of public funds can’t guarantee successful pavilions. Saudi Arabia and the United Arab Emirates created especially expensive buildings; the former with (reportedly) the largest IMAX screen on earth and 150 date palms on its roof, and the latter designed by no less than Foster + Partners. Neither is particularly exciting. Spain built a large pavilion covered in oversized wicker baskets, a less sophisticated cladding scheme than the one of earthenware hexagons conceived by Foreign Office Architects for the Spanish pavilion at the 2005 expo.

With daily crowds in the hundreds of thousands, the Expo is attracting 10 to 20 times more people than even the most robust pavilions can accommodate. Many of the pavilions were conceived as shells, with the exhibitions as afterthoughts; the architects were free to create sculptural forms without being weighed down by programmatic requirements. To an architecture critic, this is troubling: Buildings need to reconcile form and function. But given the beauty of the seed cathedral, does it matter that it only holds 100 people at a time? Who remembers what was exhibited in the Trylon and Perisphere of the 1939 New York World’s Fair, anyway? As past expos have shown, it’s the iconic forms that linger in the imagination.

Some of the smartest exhibitions in Shanghai can be found in corporate pavilions. (This shouldn’t come as a surprise, given the success of GE, IBM, Kodak, and others at the 20th century U.S. world’s fairs.) This time, Chinese companies are in the spotlight. One of the boldest, Broad Air Conditioning, devoted its pavilion largely to the 2008 earthquake. There’s a diorama showing lifelike figures buried under rubble, and a ride that simulates the quake itself. The goal is to shock provincial Chinese into demanding higher-quality buildings: a noble end, even if the means are a bit jarring.

Ironically, it is the U.S. pavilion (yes, that again) that seems to be compromised by financial interests. Though it purports to be a national pavilion, it is really an advertisement for the multinational corporations—including Chevron and Dow Chemical—that sponsored it. The people waiting in line for hours will be disappointed when they get inside, only to view a ho-hum video in which corporate spokesmen share a screen with Hillary Clinton and President Barack Obama (as well as a child who announces that cars may run on fruit juice some day).

To see impressive U.S. technology, visit instead the Shanghai Corporate Pavilion, nicknamed the “Dream Cube”—a filigree structure composed of 40 miles of LED-laced acrylic tubes, by Edwin Schlossberg, the New York exhibition designer. Inside, it turns a story by the ancient Chinese philosopher Zhuangzi, who dreamed he was a butterfly, into a dazzling interactive attraction. Too bad most visitors won’t know it was created by Americans.

At the Shanghai Expo, scores of countries with far more limited resources than the U.S. make far, far better impressions, signaling their commitment to international cooperation, along with their ingenuity and can-do spirit. Sadly for the U.S., it may be a blessing that world’s fair buildings are meant to be temporary.