Jan van Leyden
Adventurer
Probably nothing... but you are already "buying" it by paying for DDI for the months leading up to the publishing of the book.
Which is why I think WotC is perfectly happy with this new model of sales, the subscription fee. Who cares if they sell less physical books... everyone is buying those books electronically via the DDI. If it means they have to print less harccopy... that's cool... no skin off their nose. And what's probably better for them using the DDI subscription model as opposed to the hardcopy sales model... is that (as folks above have stated) they KNOW on a monthly basis how much money they are bringing in.
Well, this is not exactly true, as the costs for a printed book are not only for the actual printing. Beyond the costs for the raw data made available via DDI subscription, they'll also have to cover art, art buying, print buying, logistics and several things more.
As long as WotC continues to publish books as well as online data, diminished sales actually mean less revenue.
I actually think that the publishing model of 4e, with printed stuff as well as a subscription model, is valid for a period of transfer only. Come 5th edition, we'll either have a subscription only model or a shameful return to printed books.
If I'd have to bet some money on he result, I'd decide for the first version.