The High Line’s levitating parkland has been so long and so rapturously anticipated that the nine-block segment that opens this week can hardly compete with its own story. The tale is a triumph of urban salvage. A pair of young preservationists falls in love with a weedy, ironbound rail bed threading its way above the streets of West Chelsea and the meatpacking district. Owners of the lots it crosses want to tear it down. Finally, through the miracle of persuasion, the elevated railway is converted from eyesore to amenity. But wait: There’s the real-estate subplot! Developers use the little park to leverage their most wild-eyed ambitions. City officials rewrite the zoning, values climb, and architects arrive from the far corners of the realm.

At this point we find ourselves with two distinct High Lines. One is a quiet passeggiata of deliberately rough design, the other a larger district of new art and fresh development. A year ago, the condos popping up along Tenth Avenue were a visible expression of consumer confidence. Cocky buyers were spending $2,000 for each square foot of as-yet-nonexistent floor space and a hundred times that much for a patch of colored canvas with which to adorn their future walls. (The world has changed; the apartments keep on coming, whether they’re wanted or not, and who knows if anyone will be buying art to furnish them?)

All this metamorphosis hangs on a short and slender thread of park. The High Line emulates Paris’s Promenade Plantée, a magical arbor that runs nearly three miles atop a disused railway viaduct, from the Bastille Opera to the city’s edge. But for now, the New York version goes hardly anywhere. At 20th Street, it hits a chain-link fence separating the current park from its future extension. You can stroll the entire open length in less than ten minutes.

The park itself is a pleasant stroke of green, designed by Field Operations with Diller Scofidio + Renfro, that revives the romance of industrial brawn. A lithe, glass-walled steel staircase hangs from the superstructure at Gansevoort Street, leading to a hole cut in the trestle. It’s a fine way to make an entrance into this Jack-and-the-Beanstalk world, where nature and design have been arranged to simulate neglect. The walkway of concrete planks blurs into the grass. Sections of the original track materialize and peter out. Wooden deck chairs on train wheels slide along the rails, like pieces of equipment left over from another era. (That wistfulness may be undermined, in the early months at least, by lines to get in. The Parks Department plans to manage the flow of humanity by issuing wristbands. Can reserved tickets be far behind?)

Though the High Line is sensitively landscaped, inventively furnished, and beguilingly lit, I suspect the designers will come to regret a few decisions. The gaps between those planks of concrete will surely snap heels, catch wads of chewing gum, and fill up with debris. The edges of the walkway that curl up to keep wheelchairs from drifting into the grass also make the able-bodied stumble.

A couple of wider sections tend to the pastoral, with verdure shielding the pathway. But the architects never lose sight of the desire to behold the city beyond and below, and at times they satisfy that urge with a theatrical voilà. At 17th Street, a section of the structure falls away to create a grandstand suspended vertiginously above Tenth Avenue. The move echoes similar hanging spaces by Diller Scofidio + Renfro: the mediathèque of the Institute of Contemporary Art in Boston, which looks over a thrilling expanse of harbor; the Juilliard dance studio perched above Broadway. This one offers a grittier vista of uptown traffic.

As the park makes its debut, the neighbors are waking up from a building binge, looking a little goofy. The fussiest of the condos to near completion is 245 Tenth (at 24th Street), by the Brooklyn architect-developers Jared Della Valle and Andrew Bernheimer. Their building aches to be hipper than it is. It nuzzles the High Line and bulges on the way up. The shape maximizes square footage, but Della Valle and Bernheimer have also declared it an abstraction of a steam locomotive’s puff of smoke, evoking ye olde freight trains. Extending the metaphor, they wrapped the building in steel panels with diamond-shaped nubs punched in a digitized cloud pattern: denser near the bottom, dissipating toward the upper floors. It’s very clever, or would be if there were a reason to reside in a simulated cloud. The arbitrary and anachronistic reference does little to solve the problems of an awkward L-shaped building wrapped around a gas station.

Willful idiosyncrasy isn’t all bad, however. The most flamboyant building of the too-late wave is also the best: Jean Nouvel’s glistering half-cylinder that sweeps around the corner from 19th Street onto Eleventh Avenue. Construction involved a dance of giant megapanels, factory-made in China and hoisted into place. Each is a mosaic of variously sized windows at an assortment of rakish tilts. The angled panes reflect light in different directions, so that they sparkle like rhinestones on an upthrust sleeve. The building is a fabulous ornament, a tour de force of glitz. Nouvel’s 100 Eleventh Avenue and Frank Gehry’s IAC headquarters now pose and preen together, the glimmer twins of the West Side, sensuously advertising the future of glass.

The strange thing about West Chelsea’s new chic is that it must contend with acres of immutable drear. It remains a mouse-gray zone. Nouvel’s bauble rises alongside broad rapids of traffic, and its adorable little balconies catch the rising fumes. With astonishing sleight of hand, Nouvel has managed to keep the apartments simultaneously protected from the street and open to the views by sandwiching sunrooms and outdoor spaces between layers of glass. The wall of windows drops all the way down to the street, and the lower stories have a second perimeter of glass panes and voids: a rhythmic alternation of nothing and almost nothing. In other works, Nouvel has mastered the architecture of desire, a play of screens and veils. Here he offers a teasing double layer of peekaboo gauziness. He has presented the city with a proud object of desire, an emblem of real estate’s timeless erotic appeal.

There’s something exhilarating, and also amusing, about the arrival of such an exuberant building in somber times, like a bride bursting into a funeral hall. Perhaps 100 Eleventh isn’t late to the last party, but early to the next one. This is New York: Excess will eventually seem natural again, and when it does, this building will be ready to oblige.

Nouvel’s building represents commercial development at its finest, but it remains at odds with the High Line, which was brought into being by activists who took the notion of grassroots quite literally. Idealistic designers restored railings and doled out stairways, and chose grasses to recall the tranquil days when the structure was beautifully forgotten. Their efforts attracted commerce, which is only to be expected; if the Garden of Eden reopened today, someone would put up time-shares just outside the gates. Maybe the recession is the purists’ revenge. The green is gone, while the greenery is just getting lush.

Photographs: First spread, clockwise from top left, Thomas Loof; Elizabeth Felicella; Kim Wendell Design; David Sundberg/Esto; Courtesy of Della Valle Bernheimer; Courtesy of Audrey Matlock; Albert Vecerka/Esto/Courtesy of the IAC building; Field Operations and Diller Scofidio + Renfro/Courtesy of the City of New York

Photographs: Second spread, from left, Albert Vecerka/Esto/Courtesy of the IAC building; Hypertecture/Courtesy of Bishopscourt LLC; Courtesy of Shigeru Ban; Courtesy of Ateliers Jean Nouvel; Courtesy of Roy Co.; Hayes Davidson, London; Courtesy of Della Valle Bernheimer; Hayes Davidson/Courtesy of Young Woo & Associates