Expedition to Castle Ravenloft is very popular, I think in large part due to its darkly charismatic, larger-than-life villain, whose motivations essentially drive the story. I think it's often a good idea to have a villain like that. Ravenloft gets you to care about the vampire's motives, and they're complex enough to be a puzzle in and of themselves. It also contains a rich, vivid setting, and the plot uses every bit of it. The players get to visit gypsy camps, underground crypts, haunted swamps, etc and it all feels natural rather than just a leisurely afternoon jaunt. Since the villain seems all-powerful and omniscient, there's also a sense of urgency, since he seems strong enough to crush you like a bug and you need to defeat him before he does.