Lestat (Tom Cruise) mentioned that he can read minds and told Louis (Brad Pitt) that every vampire has a dark gift and the dark gift is different for all of them. But Louis's dark gift wasn't revealed in the movie. What was his dark gift?
He meant that the degree of that power (and when it manifests) varies. From the IMDb FAQ:
When someone is given the Dark Gift, it means they have been transformed from human to vampire. The Dark Gift IS vampirism. In the Ricean universe, all vampires are granted the same abilities: telekinesis, pyrokinesis, telepathy, and mesmerism, along with enhanced senses, strength, and immortality. What was unique to each person was the degree to which a fledgling would manifest each power immediately after being born to darkness. For example, when the Dark Gift was bestowed upon Lestat, he quickly developed a high degree of telepathic ability, whereas Louis did not.
Yes Louis's gift was his humanity, he retained his human soul - feelings emotions, and although very painful for him, empathy. Hardly a gift one might think. However, we often see that vampire characters become jaded with their unfeeling unchanging existence & inability to feel. So perhaps it was a gift.