When customers visit an ecommerce website, there are certain expectations they have from their experiences with shopping on other sites. Here are some of the features all ecommerce websites should contain:

Product Catalog - Including an easy to use hierarchical navigation
Search - An ecommerce site must have a robust search engine
Shopping Cart - One that saves the contents if the user leaves the site.
Secure Checkout - Customers expect to see that little lock at the bottom of the browser window
Order Status - Your customers should be able to log onto your site and check the status of their order

And this is just the basics. Here are some of the optional ecommerce features:

Customers also bought - Other people that bought this item also bought these other items
Top sellers - These are items that have recorded the most sales
Featured products - Show select products on your home page
Sale items - Allow for products to go on sale and change the price for some period of time
Coupons - Customers enter a coupon code and get a discount off their order

And the list goes on and on.