I just had an issue creating an automated task. (Yes, I KNOW I said NEVER ever create Automated tasks in Tasks, Activities, Notes & Attachments, but this was for a legitimate reason - well probably I could just create an email notification, but they can be ignored also - at least with a Task they have to complete the task also. The legitimate reason was that the web form was updating Salesforce and I need a notification to the record owner when the web form is complete).
The Notifiy Users Checkbox was not available on the Tasks screen on the Workflow. Now I don't think I would have ever noticed it, because I so very rarely use this feature, but I had a complaint from the users that they were not getting notified. But the Notify User checkbox was visible when I click Tasks from the Workflow menu.
Hmm, it's a new feature http://docs.releasenotes.salesforce.com/en-us/winter15/release-notes/rn_sales_activities_notifications_task_assignment.htm
You can check a setting that allows users to determine whether they get notified for tasks or not. WHAT? You have set up tasks, and then the users can elect to not receive email notifications for them? Come on! That is ridiculous. That just shows you are not using tasks well at all. (Yes, I KNOW users will just send them to junk in their email, but that is another story).
Anyway, if you care about your tasks, uncheck this setting so you can continue to set the setting in the workflows.
Or tell me in the comments on what basis you would use this feature.