The Divine Comedy was not intended to be a definitive work on the geography of the underworld; instead it is a work on ethics, designed to elevate its readers to happiness through virtuous living, according to a letter of dedication that Dante wrote to his patron. The poem is also deeply symbolic; thus in the first canticle, the Inferno, sins and their punishments take on a prominent role, where the former can be thought of as representing states of the human soul in its decline. Dante's Hell is a place of eternal punishment for sins committed in life, and the beautifully just logic of this punishment is that it is no more and no less than the sin itself: within every sinner being punished is "observed retaliation", in accordance with the nature of the wrongdoing (Inferno XXVIII, 142).
The notion of time passing (or not passing) in the Inferno borrows heavily from Saint Augustine's conception of eternity; that is, all moments -- past, present, and future -- exist in a single moment, for every person. Dante's afterlife is no mere continuation of life; existence in Hell is outside temporality, which makes this type of punishment possible, and even worse than any temporal punishment that one might have suffered through during life: it is impossible to hide from one's sins when they are always in the forefront, and always happening in the present (such as it is). It is this awareness of eternity that underpins the entirety of the Inferno and makes its punishments so appropriate.
The vestibule of Hell, situated in front of the Acheron, is inhabited by those people who "lived / Without occasion for infamy or praise" (III, 35-6). Among them, and perhaps best exemplifying their plight, is "the shadow of that man / Who out of cowardice made the great refusal" (III, 59-60). This is most likely Pope Celestine V, who abdicated from office on the false advice of a cardinal; however, he is not named explicitly, nor is anyone else in the vestibule. They remain nameless precisely because they took no action; this failing ensures that "the world does not remember them at all" (III, 49). Denigrated as "miserable and useless" and described by Dante as a "calamitous crowd, who never were alive", the punishment borne by the denizens of the vestibule accords precisely to their actions (or lack thereof) on earth: having done nothing of note, they are doomed to spend all of eternity with "mercy and justice [treating] them with contempt" (III, 49-64).
The vestibule's physical location in relation to the rest of Hell is also an important factor to consider; those who are consigned to remain there for eternity are on the shore of the Acheron across from Limbo and the rest of Hell proper. The implication of this last is that they are unworthy even of the attention of Charon, the ferryman, or Minos, the judge of damned souls; their indifference to others or to the circumstances surrounding them in life is matched by the indifference of others to them in death.
The wood of the suicides is another example of a sin being perfectly matched with a punishment. Suicides are placed by Minos into the seventh circle of hell, which is reserved for those guilty of sins of violence -- this is hinted at by the guardian that stands between the sixth and seventh circles, the Minotaur, whose nature is bestial and violent, "the infamy of Crete" (XII, 12). Suicide, of course, is a crime of violence against oneself; thus it is logical that those guilty of it be placed here. To accord with the different types of violence -- against oneself or one's property, against others, against nature, against God, and so on -- the circle is divided into rings; the second of these is reserved for those who committed violent sins against themselves. It appears as a dark wood with no evident path running through it, filled with trees "not green, but of dark colour" with "branches not wholesome, but knotted and twisted" (XIII, 2-5); and in fact the trees themselves are those damned souls guilty of suicide, grown "into branches and woody plants" to be tormented by harpies as punishment (XIII, 100).
What brings this rather odd-seeming punishment into perspective as eminently appropriate to the sin of suicide is a comment from the damned spirit of Pierre della Vigne, whom Dante and Virgil encounter in the wood. He reveals that his lot, and that of the other men turned to trees, is to "look for [their] mortal bodies, / But none [...] will ever put his on again", because "it is not just for a man to have what he takes from himself" -- that is, after having taken one's own life it is not at all fair for one to expect that it be given back (XIII, 103-5). Seeking to regain what one has taken from oneself would be painful enough; but the shape that the spirits are forced to assume is bitterly ironic in itself to add insult to injury. Della Vigne says that the bodies of the suicides "will be hung, each one upon / The thorny tree of his tormented shade", conjuring up images of the gallows on which he might have ended his own life (XIII, 107-8). But it is not until a second spirit speaks to Dante and Virgil about his fate that the appropriateness of the punishment becomes perfectly clear: in ending his life, he says, he "made a gallows for [himself] of [his] own house" (XIII, 151). Thus, the spirits in the wood of the suicides have been transformed into the very means of their own self-destruction, and are forced to remain that way forever.
The hypocrites, in the sixth ditch of the eighth circle of Hell reserved for those who engage in fraudulent activities, are also condemned to wallow perpetually in their sins; only in this case they are forced to wear heavy cloaks, the outsides of which are "gilded so that they are dazzling; / But inside [they are] all lead" (XXIII, 64-66). This is hypocrisy made visible; the glittering outsides of the cloaks are nothing at all like their linings, and the gilt serve to hide the dull ugliness of the lead, just as the hypocrites spoke one thing and meant or did another during their lives. Further down, in the ninth chasm of the eighth circle, is Bertram de Born, who was judged to be "guilty of causing discord" (XXVII, 136). His sin was destroying the relationship between a father and a son through providing misleading and "evil advice" (XXVIII, 135). His punishment is having his head separate from his body, forced to carry it "like a lantern"; his body and head are "two in one and one in two", in death subject to the same schism that he incited in life. (XXVIII, 124-5)
The punishments for the rest of the sinners in Dante's Hell follow along these same lines; in one way or another, the retribution is equal to the sin that made it necessary. But this Hell is not solely the realm of sinners; the virtuous pagans that inhabit Limbo, across the Acheron from the vestibule, must also be dealt with in some way. Dante's guide Virgil is one of these; describing the pagans he says that "they have committed no sin, and if they have merits, / That is not enough, because they are not baptised" -- for without having been baptised, one cannot enter into heaven as a Christian (IV, 34-5).
Through no fault of their own, by dint only of having been born before Christianity, these souls are confined to Limbo "to live [...] without hope, but with desire" (IV, 42). This, too, is a difficult fate, to be sure -- but it is still a fair one, as the pagans are not being punished for their lack of belief in Christ. Also, those pagans who would otherwise have found themselves far further down in the pit of Hell for their misdeeds were treated with lenience because of their pre-Christian status; for example, Dido, the queen of Carthage who took her own life, is to be found in the circle of the lustful rather than the wood of the suicides because there was no precedent for suicide as sin -- nor indeed was the idea of "sin" itself extant -- when she lived and died.
The perfect justice that can be observed in Dante's Inferno comes about because of ascribing punishments that are equal in nature to the sins that brought the souls to Hell in the first place. Every one of those in the pit of Hell is getting exactly what he or she wanted in life; the only difference is that in the afterlife it is constant and perpetual, with no chance at all of reprieve.
Dante. The Divine Comedy, trans. C.H. Sisson. Oxford: Oxford University Press, 1998.