The Midsummer Marriage, Royal Opera House, London

Fifty years is a long time in the life of some operas. In the case of The Midsummer Marriage, it's an eternity. Since its premiere at the Royal Opera half a century ago, the perception of what an opera is or should be has shifted. In the past decade, it's been a seismic shift.

So we really have to ask ourselves why, in the centenary year of Tippett's birth, his most celebrated opera should be far from full on its opening night and - more significantly - even less full at the start of its final act. Far from embracing it, people are walking away from it. Why?

Because, as opera, The Midsummer Marriage has insurmountable problems. It might even be unstageable.

It's hard - no, impossible - to reconcile Tippett's verdant, effusive and often ravishing score to the proto-new age tosh of his libretto. It really is a stinker: dense, obtuse, unsingable nonsense. There's even a line saying: "Now is this nonsense at its noon." Too right.

What is it about? Well, that part is simple; it's about the mating game. And if that sounds like the title of a sitcom, I think it once was. It's about love, it's about sex, it's about renewal, it's about the generation gap, the old and the new. It's about rebirth.

Yet even literary minds (and I canvassed one or two) emerged with furrowed brows after each act. What chance the new and younger audience this opera purportedly celebrates? Who is it really for? And where were Tippett's editors when we needed them?

Someone should have stopped him from writing his own librettos. I know he hoped TS Eliot might be persuaded to take this one on, but that could have been even worse. What opera needs more than anything are words which release the music, not inhibit it. And - dare I even say it - it needs drama. I could accept (grudgingly) all the symbolistic claptrap if The Midsummer Marriage weren't so dramatically static. And for so long. This is not an opera, this is an oratorio.

The director Graham Vick (for whom I have a lot of time) was/is plainly defeated by it. His 1996 staging (now revised and developed, apparently) is feeble. If you go down to the woods today you're in for a big disappointment. Vick's picnicking ensemble encounter "ancients" straight out of some Shaker folk-fest. There is a spiral staircase connecting hell and heaven (now that's original); there's a mystical orb representing the womb and the world from which - in the opera's only dramatic highlight - emerges just the head of the mystical soothsayer Sosostris. Too bad the wonderfully rich stentorian tones of Elena Manistina were so muted by the heavy veiling. I'd sue if I were her.

But let's hear it for the Royal Opera Chorus (thrillingly incisive throughout) and a wonderfully vital young cast. Amanda Roocroft's Jenifer negotiated her vaulting vocal lines with engaging aplomb while her lover and would-be husband Mark was energetically and ecstatically sung by Will Hartmann, a German with perfect English, though he might just as well have been singing in German. Bella and Jack (the Papageno/Papagena pairing) were blessed with a similar enthusiasm courtesy of Cora Burggraaf and Gordon Gietz. And there was also the boomingly authoritarian King Fisher of John Tomlinson.

So what to take away from this long and dispiriting evening? The zest and dappled beauty of Tippett's orchestral writing, of course. But then again, for all the energy of Richard Hickox in the pit, the glorious Ritual Dances of Act II were so successfully hijacked by Ron Howell's risible choreography that one wished Tippett had never added the word "dances".

At the close, the "womb", blossoming like a pink icing-sugar flower from some erstwhile Busby Berkeley revue, opens to reveal Jenifer and Mark impatient for their wedding. They were not alone.

To 18 November (020-7304 4000)