When you become a vampire, you now have a bloodsucking animation and attack. It's no longer some crappy drain life spell. As time passes and you don't feed, your appearance begins to detiorate. If you feed at least once a day, you will stay normal looking. As a result, people will treat you normally and won't know you're a vampire. In Morrowind, as soon as you became a vampire, people shunned you and wouldn't even talk to you. Now it's possible to stay among the living. IAlso if you stay well-fed, the sun won't harm you at all.
Perhaps the best feature is that the longer you go without feeding, the more skills and stats you gain. If you go one day without feeding, you earn Vampire Sight, which is a mix of nightvision and detect life (see people's auras through walls and such). If you miss 2 days, you gain a high level charm spell to trick your prey. If you miss 3 days, you gain the Reign of Terror, which silences and demoralizes all the people around you. If you miss 4 days, you gain the ability to be invisible for 3 full minutes, as well as nighteye for 1 min and half. In addition, your stat bonuses increase with time. It goes from +5 to +20 for most of your skills and stats. That's a major boost.
