I think a great addition to nopCommerce would be a plugin for email services to handle the newsletter subscriptions and marketing such as MailChimp.com or ConstantContact.com. Many other shopping cart systems already have them available as you can see at this link - http://www.mailchimp.com/plugins/. I have had the request more than once from a client that they would like the integration built in, instead of having to do an export from nopCommerce.
The problem with the built-in newsletter features is not really fit for large lists. We have a contact list of over 11000 email addresses Just try to send a mailing for that using regular email and I can guarantee you'll get blacklisted in no time... If you want to send high-volume newsletters you need to use specialised provider like campaignmonitor or mailchimp...