Specifically, Kozmo.com was founded in an era where you could IPO without having profits. Having that mentality from day one allowed them to make huge concessions to users such as give them free delivery on everything, and not have a minimum order. For example, you could pay $1.50 for a gum on Kozmo and get it delivered to you within an hour. WebVan, on the other hand spend $1B on building it's own warehouses and fulfillment infrastructure.
Learning from those companies, we have done a lot of things differently. For starters, there is a minimum order of $10. There is a delivery fee of $3.99 for 3 hours and $9.99 for 1 hour. (Would you not pay $4 for someone to do all your groceries?) And, we do not hold any inventory - all of it is sourced directly from local retailers.
It is also important to mention that the time that we live in is very different. People are a lot more comfortable adding their credit card information on web/mobile. Not to mention, the access to smart phones that people have gives customers the ability to shop from anywhere - office, couch, next to the fridge.
We believe we are different from the companies that have tried this in the past. And, we hope we are live in your hometown very soon.
(edits to follow)