It is the strong single player campaign that Corneliab just mentioned, that keeps us going back to pretty much every Castlevania to date (except Judgment).
We keep going back and talking about the mythos, the references, and the story, trying to piece things together, based on that single-player mode.
There are very very few games set up like Castlevania (adventure/platforming) in which people rave about the multiplayer mode, except maybe the Mario games (and that's a Huuuuuuuge maybe).
We've been going back to all the older CV games for years, but I can guarantee you that once the (already dwindling, based on the responses on this form) Fame of Harmony of Despair fades, you won't even be able to find partners to play with you through the game, 'cuz no one will really care. Do you really think, going down the line, in five or six years, that you'll pick up the game and find people actually playing it? Color me skeptical, but it might be just about the same number of people who are on, right now, in OoE or PoR sharing their Shop Codes and having their game shop open.
Then again, I could be wrong, but based on the evidence so far, I don't think I will be.
Again, who knows? I suppose it 'could' be a great game, but I wouldn't put my hopes up on it.