On Saturday evening, the Mostly Mozart Festival finished its breakout season with an unfinished work. Mozart died before he could sign the Requiem; it has come down to posterity with the changes and additions of Franz Xaver Sussmayr and been accepted as complete for more than 200 years. It might have been even more effective as a thanatological essay, however, had it been left in its original, interrupted condition, like the C Major Piano Sonata of Schubert or the Ninth Symphony of Bruckner. One fact is incontrovertible: This is the most personal music ever penned by the composer.

On Saturday what Leonard Bernstein called the pity and power of the piece was explored by maestro Louis Langree, who is virtually single-handedly responsible for the marked improvement in the festival’s orchestra. This entire festival has been notable for exceptional instrumental tightness. It has served notice that, in the future, the summer programs at Lincoln Center are to be taken seriously. In this majestic interpretation, Mr. Langree whipped his forces into a frenzy for the fire and brimstone parts. The phrasing was broad and eloquent, the attacks sharp and deep. This was a big, modern Mozartean sound, perfectly fitting such a remarkably cosmic rumination.

In the Confutatis, the Riverside Choral Society created images as frightening as any in Tintoretto’s “Last Judgment” and seemed fully invested in their enthusiastic performance. Expertly prepared by director Patrick Gardner, their body language alone was enough to convince this reviewer of their sincerity. If I had a quibble with these exciting passages, it was that the chorus could have enunciated with more volume. But I will take good intonation over decibel level any day.

So the power was certainly there, but what of the pity? Here the performance was not as satisfying or as successful. Some of the beauty may have been glossed over with brisk tempi; I wished to linger, for example, over the Voca me. The soloists – Lisa Milne (soprano), Jill Grove (mezzo), and Christoph Pregardien (tenor) – were neither spectacular nor forceful, but each possessed a clear and pleasant tone and meshed quite well with bass Nathan Berg, who was a cut above (although several octaves below).

Mr. Berg anchored their quartet singing in what proved to be the highlight of their evening. It was fascinating to compare the rather wan individual sounds of the three higher voices to the rich ensemble totality. Mr. Berg’s Tuba mirum was especially noteworthy, both for its technique and its raw, stentorian character. He is destined to create a bone-chilling Sarastro some day.

There is much to be praised in an interpretation of the Requiem that emphasizes the fear of the afterlife, leaving its listener chastised and sobered. Old Bach may have been all but forgotten at the end of the 18th century, but

Mozart, who studied with his son in London, was well aware of the master’s exceptional ability to move people through the dark, labyrinthine complexities of the fugue. Mr. Langree pulled out the stops in the Sanctus to remind us of this vital connection in the history of music.

Overall, this stunning though less than perfect performance left the audience rapt. Seldom does a reading produce that best of all possible reactions: a long, respectful silence followed by a prolonged ovation. This festival ended on just such an electrifying note.

The evening began with a serviceable but lackluster traversal of the Concerto Number 25. Soloist Jeffrey Kahane makes his living as a conductor and pianist, but my guess is that, when he looks in the mirror, he sees instead a composer, since he only seemed to awaken from his lethargy when diving into a spirited read-through of the cadenza – a decidedly unMozartean compendium – that he himself wrote for the piece.

Mr. Kahane’s keyboard technique is fraught with problems, not the least of which is uneven dynamics within individual phrases. Further, his somewhat tinkling sound and many inaccuracies formed an especially weak background for the many dissonances in this particular concerto, put there by Mozart to inspire some expansion of the Classical ear. In Mr. Kahane’s version, they simply sounded like additional mistakes.

So the 2004 Mostly Mozart festival marches into history and will be remembered as a watershed. A new orchestra with a new contract, a solid leader at the helm and bags full of enthusiasm – all augmented by a courageous and varied set of programming ideas. The future looks bright indeed, but there is still much work that is, as yet, unfinished.