Originally the incentive was the free digital copy that came with the paperback, but that freebie never went away and we decided to make it a permanent bonus. Right now I'd say the pre-order incentive for paperbacks is higher shipping priority, since we send them out based on when the order was placed. So for instance while the people who pre-ordered
Kira Kira early are now getting their copies in the mail, you'd end up at the end of the shipment queue if you bought the book now and have to wait for us to send out all the pre-orders before we can send you your book.
As for digital copies, the incentive was (and I suppose still is) that you get to read the chapters ahead of time. Take
the new Key book as an example. The way it was originally planned, the pre-order would have started last week, and the book would've been released 3~4 weeks later. We experimented a bit with the first few titles, and instead we ended up with the current system where we put out the first chapter a week after the pre-order goes up, and the rest over the course of 1½-2 weeks. You still get the full book at around the same time, but pre-ordering gives you access to the chapters earlier.
I don't know if this was the answer you wanted, but basically we already have pre-order incentives. They've all just become permanent features.