I am currently listening to a good new recording of Mozart's Mass in C minor, K. 427. Known as the "Great" Mass because of its length and scope, it's Mozart's greatest religious work, and his second-most-popular religious work after the Requiem. What it has in common with the Requiem is that both pieces were left unfinished. Mozart wrote and premiered the Great Mass in 1783, but he only wrote half the Credo and didn't write any music for the final section of the Mass, the Agnus Dei. (He also left the orchestration incomplete for a few passages.) As the liner notes to the recording explain, nobody knows why Mozart didn't finish the Mass, or what music he used for the missing sections when he performed it. And since the movie Amadeus did not provide us with a made-up explanation for all this, we may never know for sure.
It's possible that Mozart didn't finish the Mass because it had simply gotten too long. The Mass as Mozart wrote it takes 50 minutes to an hour to perform; if he had finished the whole thing, it would have been as long as Bach's Mass in B minor or Beethoven's Missa Solemnis -- a huge work that's too long to be performable as part of a church service. The last part of the Credo that Mozart finished setting, the "Et Incarnatus Est," is a huge, nine-minute soprano aria in the style of the soprano arias from Mozart's operas, including his favorite device of accompanying the soprano with woodwinds that mimic her musical phrases; the other soprano aria, "Laudamus te," is an extremely difficult coloratura calling for phenomenal breath control (that the singer on this recording doesn't really have). And in between these grand-opera arias are numbers written in a "learned," pseudo-baroque style, even including a direct quotation from Handel's Hallelujah Chorus. It's a strange and wonderful mixture of styles that suggests that Mozart is trying to incorporate all his influences -- Italian opera, baroque religious music, popular tunes -- into one piece. You could even speculate that he didn't finish the work because, having written such a hodge-podge of styles into the work, he couldn't decide what style to pick for two of the climactic sections of the Mass (the end of the Credo and the end of the Mass itself), and finally gave up trying.
The tricky thing about the text of the Mass is that unlike most of the texts that are the basis for choral works, the Mass does not really have a built-in dramatic structure to it. The Requiem is a drama; it has an emotional arc to it, progressing from mourning ("Requiem aeternam") through fear ("Dies Irae") through other emotions, finally to a compromise between fear and hope ("Libera me, domine"). The Mass is more impersonal: most of it consists of either praise ("Gloria") or religious doctrine ("Credo").
Some composers have found ways to personalize the text by making it more about people than about metaphysics; Haydn's late Masses often included musical allusions to war, death, and even sex: one of his masses contains, at the world "Qui tollis peccata mundi," a self-quotation from a love duet for Adam and Eve in "The Creation," making it clear that sexual relations are not the sin that God is being asked to take away. Haydn also came up with the idea, in some Masses, of treating the final line, "Dona nobis pacem," as a loud and even angry demand for peace rather than a prayer. Haydn's pupil Beethoven would take these ideas and expand on them in the Missa Solemnis, which is basically a liturgical opera, using the text as a jumping-off point for a musical portrait of how human beings relate to God, the world, and each other.
Mozart's "Great" Mass is sort of in-between the non-dramatic Mass (the kind of short, de-personalized Mass for an aristocratic church service, which Mozart had written many times before) and the dramatic, operaticized Mass. Because he didn't finish it, and didn't decide whether he would approach the finale as a prayer or a demand, we'll never know for sure what kind of work he thought this Mass should be. But that's what's interesting about a great unfinished work; it keeps you asking what point it's making, and leaves the question forever open, with only the knowledge that whatever the artist was trying to say, he was clearly building up to something really fascinating.