This might very well be the best module I have ever played, run or mined for ideas. I remember buying this at my then-FLGS (Waldenbooks remember) and then giving it to my then DM and saying in no uncertain terms "run me in this". I went through it, nearly died, lost a lot of levels and got my ass handed to me many times by Strahd, but in the end I got out, Sunsword in hand.
Since then I have run it many times. I will go as far as to say it is one of the few modules I have completely memorized.
Ravenloft was TSR's great experiment. Take the central monster and make him a fully realized character. Seems odd to ask to do this now, but back then, that was crazy talk. Gothic Horror in Heroic Fantasy? Crazy! But it worked. Sure, Strahd can be thought of as a poor man's Dracula, but he has since become his own monster.
When I got to college and switched over to 2nd ed Ravenlof became my campaign world. I had everything. All the boxed sets, all the modules. Everything. I never got to run it as much as I wanted too, but it more or less became my defacto world. I even bought most, if not all, the Ravenloft novels. Looking back it is amusing to see names like P.N. Elrod, Christie Golden, Tanya Huff and Laurell K.Hamilton among the authors.
For 3rd Edition I did get the Ravenloft books from Sword & Sorcery Studios/White Wolf and expected them to do, well, more with it. I still enjoyed the books, but I didn't get all obsessive about it like I did before.
I also picked up "Expedition to Castle Ravenloft" too, but was fairly disappointed.
With 4th ed, we now have Ravenloft all over the place. The Ravenloft board-game (which is awesome), Ravenloft as part of the Shadowfell (which is really cool) and now the Heroes of Shadow book which is full of Ravenloft inspired ideas.
Regardless of the system. Regardless of the world around it though Ravenloft is about adding a bit of darkness to your game. To take that love of Hammer horror and mix it in with the likes of Conan, Elric, the Grey Mouser and Merlin.
I also played Ravenloft using other systems such as Black Rose, a Ravenloft/True20 mix and even tried doing it under Unisystem a couple of times (WitchCraft works REALLY nice for it, but I also like using Ghosts of Albion).
I'd love to run it for my kids someday. I am not sure how well it would work under 4e, since it is more of a "mood" module than a combat one. Maybe a one-shot special using OSRIC or True 20 or even Unisystem would work out.