I've found that the option to "Send e-mail when ownership is assigned" isn't available in SharePoint 2013 (enterprise) task lists. I've found that this functionality is now included the 'issue tracking' list template.
I'm unsure why this very useful feature has been removed from the task list template. I’ve tried adding the task content type to an issue list; this works fine and the notification is sent. However, the new ‘task aggregation’ feature in mysite only picks up items in an task list, so those items stored in issue lists get ignored. Fairly frustrating.
I wonder whether anyone from MS can fill me in on when this feature has been removed?