I could go on and play the Devil's Advocate, for lack of better words, but I'm not going to. The fact of the matter is that even if Diablo 2 looked and sounded like the first Ultima, it would still be worth playing. I hate when that happens. I really do. I'm not kidding you.
Repetition is bad.
If you played the original game, you can probably figure out the story of Diablo 2 before actually playing it. Our hero from the first game, The Wonderer, is a hero no more, and is possessed by nasty demon stone. He goes around battling this force from within, destroying the town of Tristam and its people. Eventually he finds a follower, who narrates the game. To tell any more of the story would ruin the game, but needless to say, it's quite intriguing, complete with a surprise ending that sets the table for a sequel.