An excellent list of 21 Cart Usability Best Practices from eCommerce Opimization.
Number ten is an interesting one:
10. Make sure shoppers can easily access their cart contents and that it can easily be modified or adjusted.
A lot of carts (and Zen Cart in particular) don’t have a notion of “editing” an item in the cart; to modify an item in the cart, you need to add a new item to the cart and delete the old one. From a software development perspective I understand why they made this decision (clue: it’s easier), but I’ve always found it aggravating.