Sometime soon, a 17-year-old kid will come to New York for the first time. She'll be from a small town somewhere in America. She'll have prepared a list of things to see: the Empire State Building, Times Square, Rockefeller Center -- and then, almost as an afterthought, the Museum of Modern Art. Saturation coverage of today's launch of the museum's massive renovation and expansion -- a $425 million, 630,000-square-foot building -- will have reached her in her almost art-free home.

She'll brave the lines and fork out her precious $20 for admission. And then, many exhausting, exhilarating, unexpected hours later, she'll come away a different person. She'll be on her way to becoming one of our great artists, or a groundbreaking art historian, or the kind of collector who founds museums -- or maybe an unusually happy small-town cop who is also a lifelong lover of modern and contemporary art.

She'll have figured out the fuss about Picasso, having traced the artist's path from tame Rose Period pictures to the terrifying "Demoiselles d'Avignon," and then out the other side to all the very different strangenesses the "Demoiselles" produced.

She'll have fallen hard for Matisse at his most colorful and decorative but realized that there's a tough core in there, too.

Pollock will cast his spell, as will Warhol and Donald Judd, but she'll also fall in love with "minor" works that professionals barely acknowledge: the wax heads of Medardo Rosso, a rare sculptor who tried on impressionist ideas; an exquisite wall relief by second-string cubist Henri Laurens; the latest knotted-carbon chair by Marcel Wanders, out on the cutting edge of radical Dutch design.

This is the visitor who matters most -- more than any cultured connoisseurs or jaded critics, with their notebooks full of niggling doubts. And the new, bulked-out, buffed-up MoMA, now celebrating its 75th anniversary, can't do wrong by our neophyte art lover: It's got more room to show more art than it has ever had before. Short of turning off the lights, there's not much that MoMA could have done to keep its fantastic holdings from making an impact.

In addition to MoMA's usual collection of modern paintings and sculptures, with masterpieces stretching from van Gogh to Eva Hesse, there are expanded spaces custom-built to show prints and illustrated books, architecture and design, photography and drawings. For the first time, there's also space -- almost an entire floor -- dedicated to a rotating display of fine art of the more recent past, from 1970s conceptualism to the latest in video and installation art.

The Modern is no longer a smallish, eclectic accumulation of art objects that you can more or less absorb in a single day. It's now an assortment of almost separate collections that happen to live under a single roof.

It's not so much that the old MoMA has really been given much more room to breathe. In some classic areas of the collection, the density of works on view has stayed about the same; some painting galleries may be even more tightly hung than they were before. It's that there's just so much more art on show. For anyone keen on really getting to know the works of art they see, MoMA is now at least a three-day visit, almost on the scale of the Prado or the National Gallery in London. The new Modern feels almost encyclopedic in its coverage of the visual arts of the last 100 years. It has become the Metropolitan Museum of Modern Art.

That serves our small-town girl very well. There's one of almost everything she absolutely needs to get to know, as well as lots of things some people might think she could do without.

As she strolls the galleries on the fifth floor, she can choose to follow the usual creation story: Cezanne begat Picasso, who begat Mondrian. But the display has been designed so she can also short-circuit, or at least complicate, such a simple narrative: If she chooses to take another route, she can see that Cezanne and postimpressionists such as van Gogh could also beget German expressionists Ernst Ludwig Kirchner and Oskar Kokoschka, with their deep involvement in the world.

Moving down to the fourth floor, which displays fine art from 1945 to 1970, she can follow the passing of the baton across the Atlantic from Picasso to Pollock -- but she can also see that it was also, and equally, passed to European artists such as Jean Dubuffet and Alberto Giacometti.

At its best, the new MoMA is like a superbly balanced, evenhanded textbook on the history of modern art.

That's also where it's at its worst.

The excitement that the Modern used to have has been diluted: That worthy textbook has replaced the gripping novel that we had before. There used to be a sense of engagement in almost every picture on the wall. You could feel that curators had really gone to bat for certain artists and their works, and too bad if that meant someone else got left out of the mix. Now there's much less of a sense of that.

Picasso's 1913 "Guitar," a wall relief of sorts that changed what sculpture could be, and which former MoMA boss William Rubin prised straight from the artist's hands, now seems just another artwork hanging on the wall, equal to all the others in the room. You have to read the wall text to discover its importance. (But on the whole, you're better off not reading MoMA's texts. Few as they are -- one or two per room, at most -- they're also some of the worst I've come across. Instead of giving information about the work at hand, they tend to tell visitors what to feel and how to think about it.)

Matisse's great 1909 "Dance (I)," rather than filling a main wall within the run of galleries, is now tucked away above a back stairwell that I'm not sure will get much use. Museums have traditionally used their staircase landings to lodge big, decorative allegories they didn't put much store in otherwise, so the "Dance" now seems to be billed as one of those. It feels as though MoMA's curators decided that the stairwell would be the right, rational place for the "Dance" -- Matisse may have conceived his picture to be hung high overhead -- but then didn't double-check that it actually looked and felt right there. In this and several other cases in the expanded MoMA, thinking seems to have replaced looking as a guide to how works should be hung.

Barnett Newman's great, and great big, "Vir Heroicus Sublimis" is given most of one wall in a gallery. Which it deserves, in my opinion. But then other important, much more delicate works by Rothko and Ad Reinhardt are hung right nearby, where they are undone by the glow pouring off Newman's bright-red work. Rothko and Reinhardt could, quite sensibly, come after this Newman in a slide show on the history of modern painting, or you could flip the page to find them in a textbook on this country's art. But they cannot actually survive beside it on the gallery wall.

The very worst example of a work killed in MoMA's rehang is Monet's great "Water Lilies." The work is a huge, three-part painting all about bathing the viewer in delicate color and light. But it has been hung in the museum's massive new atrium, a 10-story, sunlight-filled box that makes it feel tiny and lost.

Monet's "Water Lilies" hasn't only lost its scale; its color is now off, too. The work is floodlit with bright artificial lights. To eyes otherwise adjusted to the blue-tinged daylight of the rest of the room, those floods make Monet's canvases glow a strange orange, as though they've had a dose of rub-on tan.

Architect Yoshio Taniguchi has said that he wanted his new building to be a neutral, unobtrusive vessel for the art. Maybe the problem is that the Modern's curators have taken him too much at his word. They've seen the free-floating, black-edged walls that Taniguchi has given them, and read them as a blank slate -- or the blank pages of a book -- that they can lay out any way they want. Or maybe they've imagined the white screen of a slide lecture, with pictures following each other at a measured pace. A museum, however spare, is really more like a concert hall, full of competing sounds that need to be corralled into a symphony: You need to make the brass explode, but make sure we hear the oboe solo, too.

Anyway, elegant neutrality comes with its own set of meanings and effects. It speaks of safety, comfort and a kind of corporate status quo, whereas most of the objects that MoMA shows were always meant to rock the boat.

Collecting and supporting modern art now has all the prestige that buying the Old Masters used to have. Liking Picasso or Rothko or even Donald Judd no longer signals that you're a Croesus with a rebel's heart, as it did when the Modern was founded in 1929; now, like wearing Hermes and Cartier, it signals that you understand the good things in life. That's why even rank-and-file billionaires coughed up to get the new Modern built. And maybe why they got a building that, for all its grace and elegance of detailing, could house any kind of deluxe art, or even high-end corporate offices.

The new Museum of Modern Art makes clear that it contains an unrivaled spread of hallowed modern masterpieces. Over the coming years, as they rethink their newfound space, curators may want to make clear that these great works are ornery, too; that these "masterpieces" are precious because of the discomfort they can cause, not despite it. That they are brilliant but also uncomfortable and mutually incompatible and jealous of the attention that they get. That they demand fierce partisanship, not merely tidy, equal-opportunity admiration.

Barnett Newman's "Broken Obelisk," above, dominates MoMA's new 10-story atrium, further dwarfing Monet's "Water Lilies," which seems lost in the sun-drenched space. Below, Picasso's "Demoiselles d'Avignon" (right rear) awaits today's formal reopening of the expanded museum.Jurgen Tesch, above, strikes a pose as he views Matisse's "Dance (I)," which has been relegated to a stairwell in the new MoMA. Below, Matisse shares wall space with Picasso. A helicopter hangs over the first floor's grand staircase. Below, a view of the main gallery addition from the sculpture garden.