Maybe the novelty will dim, and the seats will get slashed, and the commuters will retreat into the stony, silent privacy of rush hour in the city. Maybe after a while, Metro will just be a subway.

But not yet.

Now it's a great Buck Rogers dream of a ride, all wild colors, swooping arches, gleam and zip and flashing lights. There was Metro before yesterday, but not like this - not with the promise of a whole city a few quick whooshes away - and the first riders descended to the Blue Line yesterday with a wary delight the subway may never see again.

They crowded shoulder to shoulder onto the platforms and burst into applause when the delayed trains shuffled in. They consoled each other, humans confronting machinery, when the farecard dispensers spat out their dollar bills. A hundred senior citizens rode from Foggy Bottom to Rosslyn, singing "Happy Days Are Here Again."

"I'm having a love affair with Metro," exulted Taffy Swanby, a government worker who took a few hours before work yesterday to visit National Airport, just for the Metro ride. There were others as pleased as she was, like Sam McCrea, an Irish clockmaker who had taken the bus from his home in Arlington and picked up Metro at the airport to head to his shop downtown.

"Isn't that beautiful?" McCrea demanded as he stood on the elevated National Airport platform, gazing off at the horizon. "Right over there is my country." He plans to be a regular on Metro, he said. "After all, it's my tax dollar working for me . . . I've been looking forward to this for 10 years."

That was the mood on most of the Blue Line yesterday morning. In an area where there are whole kworlds that rarely mix, where the lines of class and race still lie across the city like invisible barricades, Washington had bought itself a giant public toy as classless in ts appeal as a Redskins.

It took Doretha Mitchell, domestic, from her home in Southeast toward her job in Fairfax County And Ed Ware, writer, from his home in Foggy Bottom to his job with the American Gas Association newsletter in Rosslyn. And Doug Birnie, policy analyst, from his home in Arlington to work at the Department of Transportation.

"For me, the subway is 10 cents cheaper for a round trip between Arlington and the office," said Birnie, who had boarded the Blue Line in Rosslyn. "And the time it takes is roughly comparable to the bus - about 40 minutes."

To be sure, the disenchanted also rode the subway yesterday, grumping all the while. Machines broke down, funneling rush crowds into long lines for farecards. Trains came in late and collected so many riders that the doors would not close, which made the trains later. And some former bus riders complained that Metro has sucked them into a morning commute they never wanted in the first place.

"The new system gives me worse service and makes me pay for it," said a Housing and Urban Development administrator, whose door-to-door bus service from Upper Massachussetts Avenue will be discontinued on July 17. His fare will jump by 50 cents, he said. "There will be more walking, and if it's raining. I'll get wet. It's no convenience, that's for sure. And it's time consuming."

But for the first day, anyway, the grumpers were outnumbered. There were riders so taken with Metro yesterday that they simply abandoned work, or school, or whatever foolishness they were supposed to have been doing, and lit out to ride the trains.

"We are recording our first Metro ride for posterity," declared Roger Sullivan, 36, an Arlington Systems Planning Corporations scientists, from behind a whirring 8 mm. movie camera. "We've been working up to this for weeks." Sullivan's 6-year old son Andy is a Metro addict, he explained, who has turned the family household into a small arsenal of Metro material, complete with the little signposts.

Andy, who Sullivan said can recite all the Blue Line Stations and spout various details in their individual passenger capacities, was beside himself. He ran back and forth on the plaform at the Eastern Market Metro station, shouting to his father, taking pictures with his instamatic."For Andy," said Sullivan, who clearly shared at least some of his son's enthusiasm, "this is a much bigger day than Christmas."

Out at Dupont Circle, two young New Yorkers had driven all the way to Washington just to catch the opening yesterday. "We're railroad buffs," explained Eddie Mckernan, 18, who just graduated from high school in Brooklyn. His friend Gregory Campolo, a 22-year-old cab driver who carries a collection of Chicago subway tokens on a safety pin in his pocket, found Metro a delight. "TV cameras and plenty of policemen," he said. "In New York, I'm afraid to ride the subway . . . there's never a cop around when somebody's mugging an 80-year-old lady."

A woman smashed a champagne bottle on the concrete wall of the Metro station at Foggy Bottom, and two folksingers burst into song. There was an unkind reference to the former owner of D.C.'s private mass transit system ("The farecard may seem complex but the citizen don't balk/Anything is better than the days of O. Roy Chalk"), which drew cheers, and distracted the champagne lady from the fact that she had cut her finger on the glass.

And all over Metro, on the cars and the stations, people talked to each other. Later for the chilly anonymity of a big subway - now it was adventure, a thing to be shared.

South of Rosslyn, a woman on the Blue Line leaned over to the man across the aisle and confessed, holding her farecard, "I have this fear, now. What happens if I've used it all up?" The man smiled and explained about adding on fare. The woman smiled too. She mentioned New York's subway. The man knew about New York and he mentioned Montreal's subway. By Crystal City the woman had moved one seat closer, and the two of them were chatting like old and very comfortable friends.