Playing a 2-hour set to a packed house at the Coliseum last Saturday right, Joni Mitchell demonstrated all the grace, sensitivity and versatility that have made her one of best-loved singer-songwriter-musicians of the decade.

Miss Mitchell's sleek, stunning appearance and delicate, fluid movements enhanced the complex beauty of her songs, adding a visual dimension that transported her concert to the realm of near-magic.

Backing her, and opening the show with a short set, were Tom Scott and the L.A. Express, the "spark" that ignited Miss Mitchell's successful conversion to jazz with the album "Court and Spark."

Scott's somewhat uneven set was received with only moderate enthusiasm. Highlighted by solos by Scott on saxophone, Larry Carlton on guitar and John Guerin on drums, the music was clouded by the Coliseum's normally muddy acoustics and by the fact that the audience was obviously unfamiliar with the material.

Oddly enough, the L A. Express seems to play with more conviction when backing Miss Mitchell than when performing their own music.

By the time Miss Mitchell appeared, the band had relaxed, and generated both more warmth and more excitement than during their own set.

Miss Mitchell's set was most striking for its flexible approach to arrangement. She drew on music she wrote for her earliest albums through tunes released on "Hissing of Summer Lawns" and even two brand-new, unrecorded numbers, but the arrangement of the recorded material all differed, sometimes subtly and sometimes markedly, from the album versions.

The neatly segued "Harry's House" and "Centerpiece," for instance, are given full jazz treatment on "Hissing of Summer Lawns," but during Saturday's performance were rendered by Miss Mitchell all alone on acoustic guitar. The stark contrast between the two versions was so subtle and startling that when she had finished, a male voice rang out, "That was beautiful, Joni!"

Miss Mitchell also went it alone, on piano this tine, for the gorgeous "Shades of Scarlet Conquering," also drawn from the new album.

"Free Man in Paris," on the other hand, was given a male-chorus-backup, greatly escalating the excitement of the original version.

Miss Mitchell moved like a sylph from piano to guitar, and from solos to full-band numbers, including, among other tunes, "Rainy Night House," "Help Me," "For Love or Money," "Trouble Child," Shadows and Light," "Talk to Me," "Edith and the Kingpin," "In France They Kiss on Main Street," and "Real for Free."

The tempo picked up noticeably as Miss Mitchell swung into the closing numbers, "Raised on Robbery," and "Jungle Line." Neither had ever struck me as Miss Mitchell's best tunes, and the recorded version of "Jungle Line," in particular, seems lifeless. But for this set, Miss Mitchell slyly snaked a feather boa around her shoulders and, with a wicked grin, urged the band to give the tunes everything they had. The set closed at fever pitch, with Mitchell fully earning the right to call herself a rock-jazz musician instead of a folksinger.

Miss Mitchell returned, to thunderous applause, for an encore. Cigarette in hand, and visibly relaxed after a long, demanding set, she sang a low-key, concert-patter version of "Twisted," including the audience in her friendly banter about human foibles.

The concert's only real flaw had nothing to do with the exquisite Miss Mitchell or the excellent L.A. Express, but derived from a curtain framing the stage and extending across the back of the Coliseum. Designed, perhaps, to create a concert-hall atmosphere, the curtain served instead to shroud the performers from the view of nearly a quarter of the audience. Many ticket holders felt, quite rightly, that they had been ripped off.