What character traits in Fortunato make him fall prey to Montresor?

It is mostly Fortunato's pride and arrogance that renders him vulnerable to Montresor's plans.  Montresor knows that Fortunato will not be able to resist the implication that Luchesi is as good a connoisseur of wine as Fortunato, and that Fortunato will insist on accompanying Montresor to his vaults to see the wine Montresor claims to have purchased.  Further, Montresor knows that Fortunato will not be able to resist the opportunity to tell Montresor that he made a mistake and paid far too much for wine that couldn't possible be the rare Amontillado.  Moreover, Montresor knows that, despite his repeated warnings to Fortunato that the vaults are too damp, especially given that Fortunato has a cold, Fortunato will proudly insist that he is fine, that the damp is nothing.  In this way, then, Montresor knows that he can manipulate Fortunato's pride to be Fortunato's downfall.