Mystery. There's a little bit of agony to anticipation that sometimes gets frustrating, while it's sometimes fantastic, the best anticipation occurs with mystery hand-in-hand. The term "mystery" reminds me of my youth, during which time I spent far too much time on Harry Potter forums & fansites, discussing every possible mystery of the HP series. I think that sense of mystery often leads to inquiry, and if you do that, there's some fantastic engagements that can take place. I believe it's Chip & Dan Heath, in their book "Made to Stick" that point out that one of the best ways to capture interest is to actually make people aware of what they don't know - think about which of these headlines is more compelling to you: "Oil Spill in the Gulf Unlikely to be Cleaned Up Soon" vs. "What's Keeping the Gulf Dirty?"