I do like the adventure (especially Expedition to Castle Ravenloft) and I think there are a number of reasons why it works:
1) Almost every room (and even many hallways/staircases) of this exceedingly large castle has something to interact with (beyond just simple combat). There's very few rooms where the PCs just move through without a second thought. Too many other modules have buildings or keeps that are just waypoints towards further locations, or else the occupancy is just random monsters to fight.
2) The Big Bad can and will appear several times over the course of the adventure, more often than not just to taunt the players... because he lets his castle do the exterminating for him. He's arrogant enough that he doesn't feel the need to get his hands dirty (until the very end.) Too many adventures don't have the party even know who the Big Bad is until the "final room", and by then there's been no reason for the group to care about the BB's demise.
3) Several items that help lead to the BB's downfall are scattered around the landscape and within the castle, giving a thrust to the group's movement forward. They aren't just traipsing around just to sightsee... there's a reason for their attentive exploration.
4) The Vistani card readings that give you different results each time not only keeps the players interested, it forces the DM to come up with new ways to get players from one place to another organically. Thus the DM can't just lay back and let the players drive the story... he/she has to always think about where they are going, how Strahd will react, and what occurs in each location after the players have journeyed there. Finding the Sunsword immediately in the Chapel will result in a much different adventure than the party needing to reach Strahd's tomb for it for instance.