THE EAST BUILDING of the National Gallery of Art is no longer new. The grandeur of its skylit hall and the richness of its details overwhelm the first-time viewer, but for those of us who know it well the joy we feel within is not unalloyed. In the nine months since it opened -- to the highest praise -- we have begun to see its flaws.

The Gallery's East Building remains one of the finest new museums in America, perhaps not as charming as the miraculous museum, designed by Louis Kahn, which Paul Mellon gave to Yale, yet better than the Hirshhorn, and better than the Lehman wing attached to the Met.

It is not, however, wholly free of architectural, esthetic and managerial deficiencies.

Critics have begun to carp at I.M. Pei's design. Some believe it now a better monument to art than a place for seeing pictures.

The building is beginning to look used. Here and there a screw has been stolen from a wall plaque, and more than 2 million visitors have left grease stains on the bannisters of its marble stairs. The acoustics of Pei's atrium have proved less than ideal. And his use of natural light does not seem quite right.

The on-schedule opening of the new museum seemed a managerial miracle, but that administrative miracle has not been sustained. Too many picture galleries once filled with modern paintings have been closed since mid-January and will not reopen soon. Though the exciting central atrium is not -- and may well never be -- a good place for hanging pictures, its extravagant design tends to overwhelm the far more mundane picture galleries nearby.

Are those glazed pyramids outside poor sculptures or good skylights? Does not the cafeteria look a little chintzy? If the "Subjects of the Artists" show has been taken down, why are those now-meaningless photographs of Rothko, Gorky, Motherwell, Newman, Pollock and de Kooning still stuck to the walls?

Griping is habit-forming, of course, and Paul Mellon might labeled it ingratitude. When the passer-by is offered a superbly installed room of superb David Smiths, shows as rich and rare as the Dresden exhibition, cut-outs by Matisse, a grand Pollock, fine Picassos, and Grandma Moses too, how dare he complain about pointless photo murals and grease stains on the stairs?

The answer is that the National Gallery of Art has set for itself the very highest standards.

Some of our annoyance may be explained away.

The handsome skylit galleries where the Mothewells and Pollocks were initially displayed have been closed for weeks, and will be closed weeks longer, because light-controlling louvers are being installed overhead. But the disappointed visitor has no way of knowing that. He walks down a long hall hung with photographs "explaining" a show no longer there, proceeding on his journey until a guard bars his way. Beyond the guard's blue uniform he sees a room for paintings with nothing on its walls. When will those rooms reopen? The guard does not know.

Other galleries nearby, those once hung with pictures by Gorky, Rothko, Barnett Newman and de Kooning, now confront the visitor with closed plywood doors.

Pei's building was designed to give each exhibition exactly what it needs, its own custom-designed floor plan and custom-designed walls. The temporary room designed by sculptor Tony Smith for Newman's "Stations of the Cross" will not be employed for any other pictures. The room built to show the "Women" painted by de Kooning is not to be used for other works of art. The same goes for the rooms once used for Rothkos and for Gorkys. Were the building to be used for a permanent collection -- a collection not yet formed -- such "down time" would be minimized, but while galleries are being built for temporary shows they will have to remain closed.

The building has two flaws that may never be corrected. One, it seems to me, is the exhibition function of its central hall. The other is its short supply of galleries provided with changing, natural light.

With its balconies and bridges, its soaring, complex space, and its always-shifting light, the central hall is stunning. But it functions as a lobby, and it does not welcome paintings.

Pictures, even huge ones like the Motherwell once placed there, look like mere decorations on its sheer marble walls. Nor is it much kinder to tapestries and abstract contemporary sculpture. Less gallery than indoor park, it seems to have been built for people, not for art. The mobile by Calder suspended overhead may ben an exception, because it moves in air.

Only statues -- works of art that pretend to be people -- seem to be at home on its balconies and bridges. Aristide Maillol's generous bronze Venus, the most successful work there, graces her surroundings. Statues of such quality thrive in crowds and sunlight. An assembly of statues, works large as life or larger, by Daniel Chester French, Michelangelo, Rodin, would prosper in that room, but these are works of art the museum does not own.

Meanwhile, the Caro, the tapestry by Miro, and colored abstract sculptures there struggle to survive.

The weekend noise that bounces off the room's hard glass and stone cannot be avoided. The room's shape, too, is awkward. It is triangular in plan, and the apex of the triangle, across from the main entrance, has little to commend it. People shun that corner, despite its towering window, because it is a place where there is nothing going on.

Of the building's picture galleries there are only three -- at the top of its three towers -- that are supplied with natural light. Filled with David Smiths, or the cut-outs of Matisse, or Pollock's field paintings, these galleries seem splendid. The galleries between them, in contrast, seem impoverished by their complete reliance on artificial light.

The slow rotation of the sun and the changing light it gives us bring works of art alive. Sculptures, under spotlights, look as if they're frozen, and the same is true of paintings. "Lavender Mist," the Gallery's Jackson Pollock, breathed when shown in daylight. Now, under artificial light, although well displayed it seems oddly still.

Gil Ravenal, the Gallery's gifted chief designer, terms the old debate between artificial light and some mix of bulbs and daylight "a philosophic issue." When galleries are flexible, when their ceiling heights and lights and walls are to be rearranged for each new exhibition, the absence of skylights is a price one pays. The price seems high.

What most troubles me about the galleries in I.M. Pei's East Building is their dearth of daylight. When he designed its spaces for big temporary shows, he did not have an option, since the rooms are underground. The same holds for the galleries in the middle of its towers, for they have rooms above them. But had the galleries just beneath its opaque roof been supplied with skylights of some sort, the building would have gained.

"It is impossible to discuss the East Building except in the light of the extravagant press coverage -- virtually all favorable -- that has already been inspired by it," observes John Morris Dixon in a roundtable discussion of the new museum that recently was published in Progressive Architecture magazine. "I would hate to be seen as a publication that feels it needs to kill the giant... (but)... "the serious question here is whether the exhibition spaces are made to seem incidental."

Critic Colin Amory, writing in Architectural Review, the British magazine, is more caustic in his comments on what he describes as "this artistic terminal building."

"The imagery is all wrong," he writes, "your baggage is not tumbling off the carousels when you reach the atrium, you are not in an airport or a shopping mall, but, believe it or not, in a space ostensibly intended for the quiet contemplation of works of art."

The most scathing -- and perverse -- attack on Pei's new museum is that of Richard Hennessy, writing in ArtForum. He feels it is, "at heart, an utterly Victorian building. All the ingredients are there -- confused and confusing spaces, greenery and skylights, grand staircases with mean denouements... a striving for effects which quickly turns pompous."

He doesn't like Washington, either. It "reeks of esthetic timidity," he writes.He wishes it had skyscrapers; he doesn't like the Capitol Dome, which, he feels, "now looms in a truly repellant, Big-Brother fashion." He doesn't like Pei's building or its "shocking funhouse atmosphere, its deeply philistine unseriousness. Airport '78," and he also condemns what he terms "the squalor" of its exhibition halls. Even the "knife-edge precision" of its stonework disturbs Richard Hennessy. "At first amazing, this ultimately disgusts."

Few who visit the East Building will be so hard to please, but even its admirers now see that Pei's museum is not without its flaws.