[theandrewbailey]

You also need to take care of:

    Catalog/pricing/inventory management
    Category/multiple product display/design
    Search
    Single product page display/design
    Shipping options
    Checkout, aside from payment (partial flow, or completely externally hosted?)
    Order management (how to get orders out of the system and into a warehouse to be packed)
    Promotions and discounts (will you have coupons?)
    Customer feedback/product reviews
    Contact me/us

And that's just the tip of the iceberg.

Source: I work for an ecommerce consultant (I use/live in Salesforce Commerce Cloud/formerly Demandware)