NEW YORK - Brooklyn used to be just a place, one of New York's five boroughs. But in the last few years it has also become a meme, shorthand for conveying all that it means to be young, urban, and hip in today's America.

It is no accident that the hit TV show Girls, the Sex and the City of the hipster generation, is set in the Greenpoint section. Or that Brooklyn-themed caps and T-shirts are edging out the Yankees merch on the shelves of sporting goods stores far from the shores of Coney Island. Or that celebrities as vastly different as Jonathan Safran Foer and Jay-Z proudly identify as Brooklynites.

As further evidence of Brooklyn rising, we now have the opening of the Barclays Center, the hugely expensive, hugely controversial, architect-designed basketball arena located on a wedge of land between Brooklyn's downtown and tony Park Slope. So many black Lincoln Town Cars were double-parked in front of its fashionably rusted steel facade for the inaugural event last month - a concert by Jay-Z, natch - that the scene could have easily been mistaken for a Manhattan bash.

Built as the home court of the rechristened Brooklyn Nets, Barclays Center is the first installment of the immense, $4.9 billion Atlantic Yards development, which could yield as many as 16 towers in the next 25 years. It also brings big-league sports back to Brooklyn for the first time since its beloved Dodgers were uprooted to Los Angeles in 1957.

Not that everyone is happy about this. Barclays' opening comes after nine years of legal struggles between the developer, Forest City Ratner, and what is often called Brownstone Brooklyn, the polyglot mix of residents who populate the borough's rebounding neighborhoods from Dumbo to Bed-Stuy. Though their dispute was ostensibly over prosaic zoning matters, such as traffic and bulk, Barclays was really a battle for the soul of Brooklyn: Would the borough remain an unruly hipster refuge - the archetypal Bobo paradise - or succumb to the sleek, sanitizing forces of Manhattanization?

The question will no doubt strike a chord with Philadelphians, who see Brooklyn as a kindred spirit. Manhattan is so physically different from Philadelphia that it is often hard to imagine its developments superimposed here, but Brooklyn is similarly composed of vast expanses of low-rise, owner-occupied rowhouses, with the middle class and the poor in close proximity.

Philadelphia's neighborhoods, particularly those surrounding Center City, have also been revived by people of the same demographic and mind-set as their Brooklyn compatriots, and they, too, struggle regularly with issues of identity, gentrification, and authenticity. So many of our planning disputes resemble the one that raged over Barclays, a consequence of big and small buildings alongside one another. With rowhouses on one side and a Gallery-size shopping mall on the other, Barclays occupies the kind of transitional zone that Philadelphians know so well. Plus the process stunk, as often happens in Philadelphia.

So, perhaps the most impressive thing about the arena design, by TriBeCa-based SHoP Architects and the stadium specialist Ellerbe Becket (now part of AECOM), is how well it balances Brooklyn's conflicting visions of itself. But as Jay-Z raps, in Brooklyn We Go Hard, "I'm a Brooklyn boy. I may take some getting used to."

It would be too much to say that the $750 million arena fits into the neighborhood, given that it occupies a Janus of a site with Atlantic Avenue on one side and Flatbush on the other, and it is wrapped in an otherworldly carapace of weathered steel. Yet the architecture manages to be both glam and gritty, foreign and familiar. It contains Brooklyn in all its multitudes.

The arena stretches low and long on Atlantic Avenue, like some prehistoric beast rising from the primordial muck. This is largely thanks to a smart decision by the architects to submerge the "bowl," or playing floor, below ground, keeping the domed roof roughly the height of the adjacent mall. Barclays' unusual, prostrate posture has already generated a whole bestiary of descriptions - whale, frog, turtle. I'll add one more: Barclays resembles a duck-billed platypus.

The beast's head is a giant ring that juts out over a paved plaza like the visor on a sports cap. The architects call it an oculus, a reference to the circular skylights often found in classical European churches. This one is more of a tornado-like swirl, intended to draw energy toward the plaza as it touches down at a single, improbable point. The architects have cleverly inserted the electronic signage on the inside face of the ovoid, thereby sparing the neighborhood blasts of colored light.

Given that so many American cities continue to hunger for downtown arenas, what Barclays' architects have created in Brooklyn is worth serious study. Most big-box buildings - convention centers, ballparks, urban malls - deaden their surroundings. Barclays energizes them.

It is one of the first modern sports arenas to take mixed-use seriously. Seven retail spaces line the ground floor and open onto the street, reinforcing the mall on the other side of Atlantic Avenue and giving people a reason to cross the street during the day. Construction of two housing towers, featuring a sizable share of affordable units, is supposed to start this winter on the arena's east side. SHoP architect Christopher Sharples, a West Chester, Pa. native, says it has become "unthinkable" to build an urban arena without a mash-up of round-the-clock uses.

Take the triangular plaza that fans out from the canopy. It will serve as an assembly area for events, but Sharples say he expects the owners to hold farmer's markets there, too. Most of the Nets' 18,000 patrons will arrive at the plaza from a new subway entrance, a grass-covered canopy that resembles a baby platypus and provides access to 11 subway lines. The building itself contains almost no parking.

Most U.S. cities would never consider an arena, even in a downtown location, without on-site parking because they can't match Brooklyn's transit options. But Barclays should encourage them to think about it, at least. It's noteworthy that the big problem during Barclays' opening concerts wasn't motorists circling the residential streets looking for parking, but hordes leaving on foot, according to a local opposition blog that has been chronicling the project.

Even more than traffic, opponents fear that Barclays will unleash a rash of slick, cookie-cutter buildings at odds with the borough's gentle scale and artisanal inclinations. The oddness of Barclays' architecture should help guard against that.

People have been scratching their heads at the oddest of those features: the building's rusted shell. It was created after criticism of the original design from Ellerbe Becket, architects of Philadelphia's Wells Fargo Center. They had proposed a glass box so bland, it might have been part of a suburban office park.

SHoP, which was then recruited to dress up the design, came up with the idea of wrapping the glass in a steel frame that had been treated with water to create a protective and permanent coat of rust. Their metal grid swooshes across the blue-glass surface, giving the arena its appealing contours, screening out the sun and making it feel like an authentic part of Brooklyn. The color references the brick and brown of the borough's houses; the rusted metal, its old shipyards.

The only section where the glass remains naked is the facade facing the plaza. This was done deliberately to take advantage of Ellerbe Becket's unique interior plan. Because the bowl is below grade, you enter at the top tier of seats, at eye level with the giant scoreboard.

This may be the arena's most innovative feature. As you walk into the lobby, you can see the flashing screen through the glass, an experience that gets the blood pumping even before the game begins. Sports arenas normally hide the action from those on the outside. Barclays allows even those without tickets a glimpse of the action.

Sitting on a bench in the plaza last week, Brooklyn writer Andrew Blum marveled at "the speed with which it has been absorbed into the neighborhood." While he sat there, he described the scene in a tweet: "Black hipsters shooting skate videos. Hasids collecting mitzvahs. Burqa stroller pushers at rest. Basically: like Brooklyn."