Enable Email Notification for subscriptions where you have a task assigned to you with a due date
Provide multiple subscription levels:
1) General subscription (notifications with any updates to the project)
2) Subscription with notifications only if you have an assigned task
3) Subscription with notifications only if you have an assigned task WITH a due date
4) Customized subscriptions on a select task level
With the current subscribe/unsubscribe method, if a user unsubscribes because they are not working on a project and then they are assigned a task, their task may be late as they will not get notifications.
On the contrary, if a user subscribes to a project, they get many notifications that are not relevant. The emails are often unnecessary and can be ignored/deleted potentially ignoring/deleting a notification that is relevant.