How does not knowing what Fortunato did to Montresor intensify the suspense?
Montresor, the unreliable narrator, begins the story by basically saying that he's had enough of Fortunato's insults; yet he never says what they were. Throughout the narrative he occasionally refers back to these insults as justification not only for his actions but at the pleasure he takes in them, but the insults themselves are never revealed to us. It does seem clear, from Fortunato's reaction once he realizes what Montresor is doing, that he doesn't understand why this is being done, which casts further doubt on Montresor's place and begs the question of just what could have been so bad as to justify burying Fortunato alive.
By not revealing the nature of the insults, we are denied the chance at anticipating Montresor's revenge, and of judging the merits of it. Had the story begun by saying what Fortunato's crimes were, we might disagree that they were worthy of vengeance, and thus we would see Montresor as an emotionally unstable psychopath. If we agreed that Fortunato was deserving of punishment, then the story becomes one of simple violence being played out, without the suspense. We might also anticipate, based on the nature of his crimes, what sort of punishment Montresor might have in store, and then argue with ourselves whether the semantics of the revenge matched those of the insult.
So, the lack of explanation or justification allows us to bring our attention fully on the emotional experience that Montresor is sharing, and it may allow us to more fully understand both characters without the impediment of judgment upon their actions.