Well, one major difference is in the character portrayals. In the graphic novel, the artists' interpretations of the characters can add a new dimension. For instance, how they draw Lestat can give a different impression compared to how he is described in the novel. The graphic novel may also emphasize certain themes more visually. For example, the theme of bloodlust can be shown more graphically through the images. And the graphic novel might cut out some of the more detailed internal monologues from the original novel to keep the story moving at a good pace for the visual medium.
The graphic novel uses visuals to tell the story, while the original novel relies solely on text. So, in the graphic novel, you can see the vampires right away, their appearance, and their actions. In the original novel, you have to imagine all of that. Also, the pacing can be different. The graphic novel might condense some parts of the story to fit the visual format better.