Apart from the physiological differences with blood between a vampire and a human, blood is also important as the sole diet of a vampire. Whether animal or human blood it is important to consume it regularly; without it, they will weaken mentally and be less capable of rational thought and action.
We are told that blood varies massively in flavour. Emmett’s preference for "grizzly" and Edward’s for "mountain lion" (TW10) may be indicative of their hunting styles and personalities, but animals don’t really appeal to vampires. The "bigger the animal, usually the better it tastes. Predators are the most tasty" (PC7), but even then it’s insufficient. As Edward explains it: "I’d compare it to living on tofu and soy milk; we call ourselves vegetarians, our little inside joke. It doesn’t completely satiate the hunger, or rather thirst. But it keeps us strong enough to resist." (TW9)
Human blood is what vampires truly crave, but even then, there is a vast range of types of human blood available, some more potent to different vampires than others. Bella’s blood smells "appallingly luscious" (TW19) anyway, but to Edward she is exactly his "brand of heroin." (TW13). She is "some kind of demon, summoned straight from my own personal hell,"(TW13) to torment him with the smell of her blood, and he later admits that she tastes "better than I’d imagined." (TW24)
This type of appetizing blood seems to be rare. Emmett has only come across it twice, "once stronger than the other,"(TW13) but unlike Edward, he succumbed to the temptation. Other vampires, like Jasper, seem to be oblivious to the differences in smell or flavour of human blood.
Carlisle is even more of a rarity. As a vampire, he is capable of dealing with human blood without seeming to experience the natural desires of the vampire. Even when Bella is severely injured by James, Carlisle is described as "methodical"(TW24) and very focused on his task.
It is also worth noting that when a vampire draws blood from a human, it usually results in a complete draining of the body. "It’s almost impossible to stop, once you get started, that’s why so few vampires are successfully created." (PC7)
James is driven "mad with thirst" (TW23) when Bella’s blood is spilled. Alice describes it as being "like sharks, in a way. Once we taste the blood, or even smell it, it becomes very hard to keep from feeding. Sometimes impossible." (TW20) She uses the word "frenzy" to describe the feeding, which gives a sense of the complete loss of control a vampire would have and why the body would be so completely drained of blood.
Edward sees his ability to stop drinking Bella’s blood as evidence of his love for her. He admits: "It was impossible.. to stop… Impossible. But I did." (TW24)