One never knows quite what to expect from a Nina Simone concert. In her sporadic club appearances over the last two years, the expatriate ''high priestess of soul'' has offered emotionally charged, often brilliantly iconoclastic interpretations of such unpromising fare as ''My Way'' and ''Feelings.''

At her concert at Town Hall (123 West 43d Street) on Friday, however, the biggest surprise was the diva's composure and good humor. Singing and playing the piano with a quartet that included drums, bass, guitar and congas, Miss Simone offered low-keyed, measured renditions of songs she hasn't performed in New York in many years.

Two themes dominated the repertory. One was an uplifting spirit of black solidarity, the other a mood of ruminative romantic nostalgia. On the political side, Miss Simone offered straightforward renditions of ''To Be Young, Gifted and Black'' and ''Mississippi Goddam,'' exhortatory numbers in which the audience exuberantly joined her for the choruses. In a softer vein, the singer also presented recent ballads written in a commercial pop-rock idiom. ''For Awhile,'' a simple, engaging waltz, written expressly for Frank Sinatra, Miss Simone used as a recurrent connective motif throughout the evening, twisting the phrase ''I'm still in love with you'' into ''I'm still in love with New York.''

The singer also reprised ''Baltimore,'' ''I Loves You Porgy,'' ''Sugar in My Bowl,'' and ''Four Women,'' songs that touch nearly every phase of her career. All were offered in a reedy, growling alto that has lost most of its beauty but none of its churning expressiveness.