The Houston Grand Opera's revival of "Show Boat," which opened last night in the Kennedy Center Opera House, is so long on virtues that you may not notice its ultimate shortcoming until the three-hour evening is winding to a close. But notice it you will, once the curtain has fallen and you run through the pleasures that have been strewn across the stage.

In this loving and meticulous production, it is the scope of things that is impressive, the majestic swell of Jerome Kern's score and the lavishness of the stage effects. The cast approaches 50, with costumes for three times that many. In waves and clusters, they come and go. One huge set supplants another. At a time of theatrical penny-pinching, when every walk-on puts a strain on the budget, this is as close to an epic staging as we're likely to see.

And yet, impressive as it looks, the show is mainly a triumph in general terms. There is really no single performer who stands out from the scenery, no principal who calls the spectacle to a sudden halt and re- minds us that "Show Boat" is a saga of broken hearts, not just a 40-odd-year panorama of life up and down the Mississippi. Although the designers have clearly eschewed gauze for their sets, the musical nonetheless unfolds as if behind veils.

Donald O'Connor is top-billed as the plucky Cap'n Andy and the show finds a moment for him at the beginning of Act II to indulge in a little tap dancing. O'Connor may have a bit of a paunch these days, but the blue eyes still twinkle and the face, jowls and all, maintains that boyish glow. But he's more in the nature of a chipper presence, nice to have around, but not the one to galvanize the proceedings.

For that, you have to look to Gaylord Ravenal, the dashing gambler, who loves 'em and leaves 'em, or to Julie, the mulatto, who's driven off the showboat when her blood lineage comes to light. Or even to Magnolia, Cap'n Andy's sweet daughter, who falls for Ravenal, only to be deserted in a cheap flat in Chicago. They are the creatures of heartache and passion. In them resides the beauty and the pain of "Show Boat."

For those roles, this production has found some pleasant performers, pure of voice and handsome of profile. But they rarely endow this revival with anything stronger than an agreeable competency. Granted, there is an undercurrent of romance, when Ron Raines (Ravenal) and Sheryl Woods (Magnolia) first lock eyes on the Cotton Blossom. But their ensuing duet, "Only Make Believe," never escalates into passion. By the time they come to "You Are Love," the tumult should be unrestrained. It isn't.

Or take the case of Julie (Lonette McKee). The show all but hands her a shining moment. Down and out in Chicago, flirting with the bottle, she perches atop an upright piano for that haunting ballad, "Bill." Everything is going for her--the angle of the spotlight, the smoke curling up from the pianist's cigarette, the backstage ambiance--but McKee, despite a certain husky charm, doesn't ever rise to the full poignancy of the moment.

You can't say that Paul Keith and Paige O'Hara are miscast as the comic song-and-dance team, but you can't say, either, that they bring down the house or even part of it with their cakewalk, "Goodby My Lady Love." In only one instance, in fact, does this "Show Boat" rise to its fabled heights and that is with Bruce Hubbard's rendition of "Old Man River." Sung with sterling sobriety and a rock-like dignity, it is the closest thing this revival has to a showstopper. It also comes very early in the first act.

As a result, this "Show Boat" scores primarily as a panorama and the performers invariably seem part of a vast unfolding freize. The book by Oscar Hammerstein II has its lurches and its potholes, but director Michael Kahn has downplayed most of them and his staging, especially in the first act, has the quality of handsome tapestry. Without forcing matters, he clearly delineates the gulf between white and black, thereby creating a distinct social context which makes a number like "Old Man River" all the more telling.

The production, as a whole, has been mounted with taste and respect, and if the length seems excessive, it is because the Houston Grand Opera has restored a scene here and a musical bridge there that tend to be amputated in more efficient productions. The whole has a certain majesterial grandeur. It's when you get down to particulars that the revival seems to lack personality. That's not to slight Karla Burns, the rotund Queenie, who has the girth to move mountains and the good nature to sweep elections. But Queenie can only do so much from the kitchen of the Cotton Blossom.

It's the lovers and wastrels on the main deck who are letting us down. Gently. Ever so gently. SHOW BOAT. Music by Jerome Kern. Book and lyrics, Oscar Hammerstein II. Directed by Michael Kahn; scenery, Helen Pond and Herbert Senn; costumes, Molly Maginnis; lighting, Tom Skelton; music director, John DeMain; choreography, Dorothy Danner. With Donald O'Connor, Lonette McKee, Ron Raines, Sheryl Woods, Karla Burns, Paul Keith, Avril Gentles, Paige O'Hara, Bruce Hubbard. At the Opera House through April 16.