As a former FogBugz User, i really appreciated the feature that you can send an email from within FogBugz which automaticly creates an issue and where the customer can reply to which is also tracked with default bug tracking standards. This way you can send an email to the customer which says "You need to buy another server for Application A" and this issue can be tracked and also replied to by the customer.

Something like all CRM Systems have. So that when you send an email from SalesForce to a customer, this email becomes part of the salesForce database as a contact item or something.