It would be REALLY nice to have an option for a custom field that sends an email notification when it is updated. This would just be adding a checkbox for each field in project settings page.

For example, we use a custom list field called "reviewstatus" that uses "review, ack, nack, etc" for approving patches. Currently we must update the custom field, and add a comment saying something "looks good, ack". So for us it would a lot easier, and more importantly less error prone, if when we update "reviewstatus" it just sends an email notification.