One feature that I liked from other carts I've used in the past was the ability to store the contents of their cart. For instance, if someone adds 'Product A' and then logs out of the system, the same product would still be loaded if they didn't log back in until 6 months (or any period of time) later.
In conjunction with that, I think it would be great if you had the ability to then view their cart and send them an email reminder that it's waiting to be purchased. If the customer was on your site, registered for an account, loaded a product, chances are they want (or wanted) it. Why not send them an email reminder saying something along the lines of:
We noticed you've shown interest in the $s at our store! If you have any additional questions that would help you make a decision on your future purchase, please feel free to contact us anytime.
Thank you for choosing $s and we hope to see you back soon!