The biggest challenge I see for amazon to come is, besides competition and makets and all that their ability to handle their whole supply chain. Amazon already proved that in terms of warehousing and distribution they are really ggod. but if there is one thing that comes with an ever increasing number of arehouses, especially if you want same-day delivery, is a ever increasing inventory level, in both number of items and value.

So from the outside one cornerstone of a strategy like this would be inventory management at at least regional if not even warehouse level. It's do-able, no question, but difficult. An advantage amazon has here is a huge history of point-of-sales data for most important regions they are operating in, no matter if there actualy is a amzon warehouse or not. And if they don't have (enough) point of sales data, well then it's to early to offer same-day service and all that.

All amazon has to do now is to keep operations up to the task... :-)