The subject matter changes. In the Inferno and in the Purgatory there is body, blood, and weight, characters who tell their own story, and punishments that can be seen; in Paradise there is light, music, dance, and number, and there is hardly any plot. Instead of scenes, what Dante finds are lessons: the order of the universe, the spots on the Moon, the value of a vow, the resurrection of bodies, the justice that cannot be measured from afar.
What is asked also changes. The Inferno shows what each sin does to a soul, and the Purgatory shows what the mountain teaches one to want. Paradise asks what a man can bear to see. That is why heaven tempers itself for the body that rises: in Saturn Beatrice does not laugh and the saints do not sing, because Dante's hearing is as deadly as his sight. The guide changes too, since Virgil stayed in Purgatory and it is Beatrice who leads, up to canto XXXI.
The difficulty grows in the opposite direction from what one expects. It is the hardest canticle and the least remembered, and it is also the one that makes the inability to tell its subject part of the poem. The form stays the same, the tercets, and so does the ending of each book: all three canticles end on the word stars.